Problem:
すなけ君は，(自己交差を持たない)
n
個の頂点からなる折れ線の形をしている．最初に，すなけ君の
i
番目の頂点は
(x
i
, y
i
)
にある．すなけ君は，平行移動や回転をして連続的に移動することはできるが，変形(折れ線の長さや二つの線分のなす角度を変える) することはできない．
y
= 0 は壁であり，(0, 0) に微小な穴が開いている．すなけ君がこの穴を通って全体が
y
< 0 をみたすように移動できるかどうか判定せよ．
Constraints
2 ≤
n
≤ 1000
0 ≤
x
i
≤ 10
9
1 ≤
y
i
≤ 10
9
折れ線は自己交差を持たない
どの三点も同一直線上にない
(x
i
, y
i
) ≠ (x
i+1
, y
i+1
)
入力は全て整数である
Input
n
x
1
y
1
. . .
x
n
y
n
Output
すなけ君が穴を通って移動できる場合は"
Possible
", できない場合は"
Impossible
" と出力せよ．
Sample Input 1
4
0 1
1 1
1 2
2 2
Sample Output 1
Possible
−
y
方向に 1 平行移動する．
(0, 0) を中心として 90 度反時計回りに回転する．
−
y
方向に 1 平行移動する．
(0, 0) を中心として 90 度時計回りに回転する．
−
y
方向に 1 平行移動する．
(0, 0) を中心として 90 度反時計回りに回転する．
−
y
方向に 2 平行移動する．
Sample Input 2
11
63 106
87 143
102 132
115 169
74 145
41 177
56 130
28 141
19 124
0 156
22 183
Sample Output 2
Impossible

Submitted solutions:

code_snippet: #include <bits/stdc++.h>

using namespace std;

#define rep(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define rrep(i, a, b) for (int i = (int)(b)-1; i >= (int)(a); i--)
#define ALL(v) (v).begin(), (v).end()
#define UNIQUE(v) sort(ALL(v)), (v).erase(unique(ALL(v)), (v).end())
#define SZ(v) (int)v.size()
#define MIN(v) *min_element(ALL(v))
#define MAX(v) *max_element(ALL(v))
#define LB(v, x) int(lower_bound(ALL(v), (x)) - (v).begin())
#define UB(v, x) int(upper_bound(ALL(v), (x)) - (v).begin())

using uint = unsigned int;
using ll = long long int;
using ull = unsigned long long;
using i128 = __int128_t;
using u128 = __uint128_t;
const int inf = 0x3fffffff;
const ll INF = 0x1fffffffffffffff;

template <typename T> inline bool chmax(T &a, T b) {
    if (a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <typename T> inline bool chmin(T &a, T b) {
    if (a > b) {
        a = b;
        return 1;
    }
    return 0;
}
template <typename T, typename U> T ceil(T x, U y) {
    assert(y != 0);
    if (y < 0)
        x = -x, y = -y;
    return (x > 0 ? (x + y - 1) / y : x / y);
}
template <typename T, typename U> T floor(T x, U y) {
    assert(y != 0);
    if (y < 0)
        x = -x, y = -y;
    return (x > 0 ? x / y : (x - y + 1) / y);
}
template <typename T> int popcnt(T x) {
    return __builtin_popcountll(x);
}
template <typename T> int topbit(T x) {
    return (x == 0 ? -1 : 63 - __builtin_clzll(x));
}
template <typename T> int lowbit(T x) {
    return (x == 0 ? -1 : __builtin_ctzll(x));
}

template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
    os << "P(" << p.first << ", " << p.second << ")";
    return os;
}
template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec) {
    os << "{";
    for (int i = 0; i < vec.size(); i++) {
        os << vec[i] << (i + 1 == vec.size() ? "" : ", ");
    }
    os << "}";
    return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, const map<T, U> &map_var) {
    os << "{";
    for (auto itr = map_var.begin(); itr != map_var.end(); itr++) {
        os << "(" << itr->first << ", " << itr->second << ")";
        itr++;
        if (itr != map_var.end())
            os << ", ";
        itr--;
    }
    os << "}";
    return os;
}
template <typename T> ostream &operator<<(ostream &os, const set<T> &set_var) {
    os << "{";
    for (auto itr = set_var.begin(); itr != set_var.end(); itr++) {
        os << *itr;
        ++itr;
        if (itr != set_var.end())
            os << ", ";
        itr--;
    }
    os << "}";
    return os;
}
#ifdef LOCAL
#define show(...) _show(0, #__VA_ARGS__, __VA_ARGS__)
#else
#define show(...) true
#endif
template <typename T> void _show(int i, T name) {
    cerr << '\n';
}
template <typename T1, typename T2, typename... T3>
void _show(int i, const T1 &a, const T2 &b, const T3 &...c) {
    for (; a[i] != ',' && a[i] != '\0'; i++)
        cerr << a[i];
    cerr << ":" << b << " ";
    _show(i + 1, a, c...);
}

/**
 * @brief template
 */

using T = double;
const T eps = 1e-8;
using Point = complex<T>;
using Poly = vector<Point>;
#define X real()
#define Y imag()
template <typename T> inline bool eq(const T &a, const T &b) {
    return fabs(a - b) < eps;
}
bool cmp(const Point &a, const Point &b) {
    auto sub = [&](Point a) {
        return (a.Y < 0 ? -1 : (a.Y == 0 && a.X >= 0 ? 0 : 1));
    };
    if (sub(a) != sub(b))
        return sub(a) < sub(b);
    return a.Y * b.X < a.X * b.Y;
}
struct Line {
    Point a, b, dir;
    Line() {}
    Line(Point _a, Point _b) : a(_a), b(_b), dir(b - a) {}
    Line(T A, T B, T C) {
        if (eq(A, .0)) {
            a = Point(0, C / B), b = Point(1 / C / B);
        } else if (eq(B, .0)) {
            a = Point(C / A, 0), b = Point(C / A, 1);
        } else {
            a = Point(0, C / B), b = Point(C / A, 0);
        }
    }
};
struct Segment : Line {
    Segment() {}
    Segment(Point _a, Point _b) : Line(_a, _b) {}
};
struct Circle {
    Point p;
    T r;
    Circle() {}
    Circle(Point _p, T _r) : p(_p), r(_r) {}
};

istream &operator>>(istream &is, Point &p) {
    T x, y;
    is >> x >> y;
    p = Point(x, y);
    return is;
}
ostream &operator<<(ostream &os, Point &p) {
    os << fixed << setprecision(12) << p.X << ' ' << p.Y;
    return os;
}
Point unit(const Point &a) {
    return a / abs(a);
}
T dot(const Point &a, const Point &b) {
    return a.X * b.X + a.Y * b.Y;
}
T cross(const Point &a, const Point &b) {
    return a.X * b.Y - a.Y * b.X;
}
Point rot(const Point &a, const T &theta) {
    return Point(cos(theta) * a.X - sin(theta) * a.Y,
                 sin(theta) * a.X + cos(theta) * a.Y);
}
Point rot90(const Point &a) {
    return Point(-a.Y, a.X);
}
T arg(const Point &a, const Point &b, const Point &c) {
    double ret = acos(dot(a - b, c - b) / abs(a - b) / abs(c - b));
    if (cross(a - b, c - b) < 0)
        ret = -ret;
    return ret;
}

Point Projection(const Line &l, const Point &p) {
    T t = dot(p - l.a, l.a - l.b) / norm(l.a - l.b);
    return l.a + (l.a - l.b) * t;
}
Point Projection(const Segment &l, const Point &p) {
    T t = dot(p - l.a, l.a - l.b) / norm(l.a - l.b);
    return l.a + (l.a - l.b) * t;
}
Point Reflection(const Line &l, const Point &p) {
    return p + (Projection(l, p) - p) * 2.;
}
int ccw(const Point &a, Point b, Point c) {
    b -= a;
    c -= a;
    if (cross(b, c) > eps)
        return 1; // ccw

    if (cross(b, c) < -eps)
        return -1; // cw

    if (dot(b, c) < 0)
        return 2; // c,a,b

    if (norm(b) < norm(c))
        return -2; // a,b,c

    return 0;      // a,c,b

}
bool isOrthogonal(const Line &a, const Line &b) {
    return eq(dot(a.b - a.a, b.b - b.a), .0);
}
bool isParallel(const Line &a, const Line &b) {
    return eq(cross(a.b - a.a, b.b - b.a), .0);
}
bool isIntersect(const Segment &a, const Segment &b) {
    return ccw(a.a, a.b, b.a) * ccw(a.a, a.b, b.b) <= 0 and
           ccw(b.a, b.b, a.a) * ccw(b.a, b.b, a.b) <= 0;
}
int isIntersect(const Circle &a, const Circle &b) {
    T d = abs(a.p - b.p);
    if (d > a.r + b.r + eps)
        return 4;
    if (eq(d, a.r + b.r))
        return 3;
    if (eq(d, abs(a.r - b.r)))
        return 1;
    if (d < abs(a.r - b.r) - eps)
        return 0;
    return 2;
}
T Dist(const Line &a, const Point &b) {
    Point c = Projection(a, b);
    return abs(c - b);
}
T Dist(const Segment &a, const Point &b) {
    if (dot(a.b - a.a, b - a.a) < eps)
        return abs(b - a.a);
    if (dot(a.a - a.b, b - a.b) < eps)
        return abs(b - a.b);
    return abs(cross(a.b - a.a, b - a.a)) / abs(a.b - a.a);
}
T Dist(const Segment &a, const Segment &b) {
    if (isIntersect(a, b))
        return .0;
    T res = min({Dist(a, b.a), Dist(a, b.b), Dist(b, a.a), Dist(b, a.b)});
    return res;
}
Point Intersection(const Line &a, const Line &b) {
    T d1 = cross(a.b - a.a, b.b - b.a);
    T d2 = cross(a.b - a.a, a.b - b.a);
    if (eq(d1, 0.) and eq(d2, 0.))
        return b.a;
    return b.a + (b.b - b.a) * (d2 / d1);
}
Poly Intersection(const Circle &a, const Line &b) {
    Poly res;
    T d = Dist(b, a.p);
    if (d > a.r + eps)
        return res;
    Point h = Projection(b, a.p);
    if (eq(d, a.r)) {
        res.push_back(h);
        return res;
    }
    Point e = unit(b.b - b.a);
    T ph = sqrt(a.r * a.r - d * d);
    res.push_back(h - e * ph);
    res.push_back(h + e * ph);
    return res;
}
Poly Intersection(const Circle &a, const Segment &b) {
    Line c(b.a, b.b);
    Poly sub = Intersection(a, c);
    double xmi = min(b.a.X, b.b.X), xma = max(b.a.X, b.b.X);
    double ymi = min(b.a.Y, b.b.Y), yma = max(b.a.Y, b.b.Y);
    Poly res;
    rep(i, 0, sub.size()) {
        if (xmi <= sub[i].X + eps and sub[i].X - eps <= xma and
            ymi <= sub[i].Y + eps and sub[i].Y - eps <= yma) {
            res.push_back(sub[i]);
        }
    }
    return res;
}
Poly Intersection(const Circle &a, const Circle &b) {
    Poly res;
    int mode = isIntersect(a, b);
    T d = abs(a.p - b.p);
    if (mode == 4 or mode == 0)
        return res;
    if (mode == 3) {
        T t = a.r / (a.r + b.r);
        res.push_back(a.p + (b.p - a.p) * t);
        return res;
    }
    if (mode == 1) {
        if (b.r < a.r - eps) {
            res.push_back(a.p + (b.p - a.p) * (a.r / d));
        } else {
            res.push_back(b.p + (a.p - b.p) * (b.r / d));
        }
        return res;
    }
    T rc = (a.r * a.r + d * d - b.r * b.r) / d / 2.;
    T rs = sqrt(a.r * a.r - rc * rc);
    if (a.r - abs(rc) < eps)
        rs = 0;
    Point e = unit(b.p - a.p);
    res.push_back(a.p + rc * e + rs * e * Point(0, 1));
    res.push_back(a.p + rc * e + rs * e * Point(0, -1));
    return res;
}
Poly HalfplaneIntersection(vector<Line> &H) {
    sort(ALL(H), [&](Line &l1, Line &l2) { return cmp(l1.dir, l2.dir); });
    auto outside = [&](Line &L, Point p) -> bool {
        return cross(L.dir, p - L.a) < -eps;
    };
    deque<Line> deq;
    int sz = 0;
    rep(i, 0, SZ(H)) {
        while (sz > 1 and
               outside(H[i], Intersection(deq[sz - 1], deq[sz - 2]))) {
            deq.pop_back();
            sz--;
        }
        while (sz > 1 and outside(H[i], Intersection(deq[0], deq[1]))) {
            deq.pop_front();
            sz--;
        }
        if (sz > 0 and fabs(cross(H[i].dir, deq[sz - 1].dir)) < eps) {
            if (dot(H[i].dir, deq[sz - 1].dir) < 0) {
                return {};
            }
            if (outside(H[i], deq[sz - 1].a)) {
                deq.pop_back();
                sz--;
            } else
                continue;
        }
        deq.push_back(H[i]);
        sz++;
    }

    while (sz > 2 and outside(deq[0], Intersection(deq[sz - 1], deq[sz - 2]))) {
        deq.pop_back();
        sz--;
    }
    while (sz > 2 and outside(deq[sz - 1], Intersection(deq[0], deq[1]))) {
        deq.pop_front();
        sz--;
    }
    if (sz < 3)
        return {};
    deq.push_back(deq.front());
    Poly ret;
    rep(i, 0, sz) ret.push_back(Intersection(deq[i], deq[i + 1]));
    return ret;
}

T Area(const Poly &a) {
    T res = 0;
    int n = a.size();
    rep(i, 0, n) res += cross(a[i], a[(i + 1) % n]);
    return fabs(res / 2.);
}
T Area(const Poly &a, const Circle &b) {
    int n = a.size();
    if (n < 3)
        return .0;
    auto rec = [&](auto self, const Circle &c, const Point &p1,
                   const Point &p2) {
        Point va = c.p - p1, vb = c.p - p2;
        T f = cross(va, vb), res = .0;
        if (eq(f, .0))
            return res;
        if (max(abs(va), abs(vb)) < c.r + eps)
            return f;
        if (Dist(Segment(p1, p2), c.p) > c.r - eps)
            return c.r * c.r * arg(vb * conj(va));
        auto u = Intersection(c, Segment(p1, p2));
        Poly sub;
        sub.push_back(p1);
        for (auto &x : u)
            sub.push_back(x);
        sub.push_back(p2);
        rep(i, 0, sub.size() - 1) res += self(self, c, sub[i], sub[i + 1]);
        return res;
    };
    T res = .0;
    rep(i, 0, n) res += rec(rec, b, a[i], a[(i + 1) % n]);
    return fabs(res / 2.);
}
T Area(const Circle &a, const Circle &b) {
    T d = abs(a.p - b.p);
    if (d >= a.r + b.r - eps)
        return .0;
    if (d <= abs(a.r - b.r) + eps) {
        T r = min(a.r, b.r);
        return M_PI * r * r;
    }
    T ath = acos((a.r * a.r + d * d - b.r * b.r) / d / a.r / 2.);
    T res = a.r * a.r * (ath - sin(ath * 2) / 2.);
    T bth = acos((b.r * b.r + d * d - a.r * a.r) / d / b.r / 2.);
    res += b.r * b.r * (bth - sin(bth * 2) / 2.);
    return fabs(res);
}
bool isConvex(const Poly &a) {
    int n = a.size();
    int cur, pre, nxt;
    rep(i, 0, n) {
        pre = (i - 1 + n) % n;
        nxt = (i + 1) % n;
        cur = i;
        if (ccw(a[pre], a[cur], a[nxt]) == -1)
            return 0;
    }
    return 1;
}
int isContained(const Poly &a,
                const Point &b) { // 0:not contain,1:on edge,2:contain

    bool res = 0;
    int n = a.size();
    rep(i, 0, n) {
        Point p = a[i] - b, q = a[(i + 1) % n] - b;
        if (p.Y > q.Y)
            swap(p, q);
        if (p.Y < eps and eps < q.Y and cross(p, q) > eps)
            res ^= 1;
        if (eq(cross(p, q), .0) and dot(p, q) < eps)
            return 1;
    }
    return (res ? 2 : 0);
}
Poly ConvexHull(Poly &a) {
    sort(ALL(a), [](const Point &p, const Point &q) {
        return (eq(p.Y, q.Y) ? p.X < q.X : p.Y < q.Y);
    });
    a.erase(unique(ALL(a)), a.end());
    int n = a.size(), k = 0;
    Poly res(n * 2);
    for (int i = 0; i < n; res[k++] = a[i++]) {
        while (k >= 2 and
               cross(res[k - 1] - res[k - 2], a[i] - res[k - 1]) < eps)
            k--;
    }
    for (int i = n - 2, t = k + 1; i >= 0; res[k++] = a[i--]) {
        while (k >= t and
               cross(res[k - 1] - res[k - 2], a[i] - res[k - 1]) < eps)
            k--;
    }
    res.resize(k - 1);
    return res;
}
T Diam(const Poly &a) {
    int n = a.size();
    int x = 0, y = 0;
    rep(i, 1, n) {
        if (a[i].Y > a[x].Y)
            x = i;
        if (a[i].Y < a[y].Y)
            y = i;
    }
    T res = abs(a[x] - a[y]);
    int i = x, j = y;
    do {
        if (cross(a[(i + 1) % n] - a[i], a[(j + 1) % n] - a[j]) < 0)
            i = (i + 1) % n;
        else
            j = (j + 1) % n;
        chmax(res, abs(a[i] - a[j]));
    } while (i != x or j != y);
    return res;
}
Poly Cut(const Poly &a, const Line &l) {
    int n = a.size();
    Poly res;
    rep(i, 0, n) {
        Point p = a[i], q = a[(i + 1) % n];
        if (ccw(l.a, l.b, p) != -1)
            res.push_back(p);
        if (ccw(l.a, l.b, p) * ccw(l.a, l.b, q) < 0)
            res.push_back(Intersection(Line(p, q), l));
    }
    return res;
}

T Closest(Poly &a) {
    int n = a.size();
    if (n <= 1)
        return 0;
    sort(ALL(a), [&](Point a, Point b) {
        return (eq(a.X, b.X) ? a.Y < b.Y : a.X < b.X);
    });
    Poly buf(n);
    auto rec = [&](auto self, int lb, int rb) -> T {
        if (rb - lb <= 1)
            return (T)INF;
        int mid = (lb + rb) >> 1;
        auto x = a[mid].X;
        T res = min(self(self, lb, mid), self(self, mid, rb));
        inplace_merge(a.begin() + lb, a.begin() + mid, a.begin() + rb,
                      [&](auto p, auto q) { return p.Y < q.Y; });
        int ptr = 0;
        rep(i, lb, rb) {
            if (abs(a[i].X - x) >= res)
                continue;
            rep(j, 0, ptr) {
                auto sub = a[i] - buf[ptr - 1 - j];
                if (sub.Y >= res)
                    break;
                chmin(res, abs(sub));
            }
            buf[ptr++] = a[i];
        }
        return res;
    };
    return rec(rec, 0, n);
}

Circle Incircle(const Point &a, const Point &b, const Point &c) {
    T A = abs(b - c), B = abs(c - a), C = abs(a - b);
    Point p(A * a.X + B * b.X + C * c.X, A * a.Y + B * b.Y + C * c.Y);
    p /= (A + B + C);
    T r = Dist(Line(a, b), p);
    return Circle(p, r);
}
Circle Circumcircle(const Point &a, const Point &b, const Point &c) {
    Line l1((a + b) / 2., (a + b) / 2. + (b - a) * Point(0, 1));
    Line l2((b + c) / 2., (b + c) / 2. + (c - b) * Point(0, 1));
    Point p = Intersection(l1, l2);
    return Circle(p, abs(p - a));
}
Poly tangent(const Point &a, const Circle &b) {
    return Intersection(b, Circle(a, sqrt(norm(b.p - a) - b.r * b.r)));
}
vector<Line> tangent(const Circle &a, const Circle &b) {
    vector<Line> res;
    T d = abs(a.p - b.p);
    if (eq(d, 0.))
        return res;
    Point u = unit(b.p - a.p);
    Point v = u * Point(0, 1);
    for (int t : {-1, 1}) {
        T h = (a.r + b.r * t) / d;
        if (eq(h * h, 1.)) {
            res.push_back(Line(a.p + (h > 0 ? u : -u) * a.r,
                               a.p + (h > 0 ? u : -u) * a.r + v));
        } else if (1 > h * h) {
            Point U = u * h, V = v * sqrt(1 - h * h);
            res.push_back(Line(a.p + (U + V) * a.r, b.p - (U + V) * (b.r * t)));
            res.push_back(Line(a.p + (U - V) * a.r, b.p - (U - V) * (b.r * t)));
        }
    }
    return res;
}

/**
 * @brief Geometry
 */

int main() {
    int N;
    cin >> N;
    vector<Point> P(N);
    rep(i,0,N) {
        double x, y;
        cin >> x >> y;
        P[i] = Point(x,y);
    }
    rep(i,2,N) {
        Poly Q;
        rep(j,0,i) Q.push_back(P[j]);
        Q = ConvexHull(Q);
        if (isContained(Q,P[i]) != 0) {
            cout << "Impossible" << endl;
            return 0;
        }
    }
    rep(i,0,N-2) {
        Poly Q;
        rep(j,i+1,N) Q.push_back(P[j]);
        Q = ConvexHull(Q);
        if (isContained(Q,P[i]) != 0) {
            cout << "Impossible" << endl;
            return 0;
        }
    }
    cout << "Possible" << endl;
}
accuracy: 1
time_ms: 50
memory_kb: 3636
final_rank: 3

code_snippet: #include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,n) for(ll i=0;i<ll(n);i++)
#define ALL(A) A.begin(),A.end()

#include<complex>
#define mod 1000000007
using ll = long long;
const int INF = 1000000000;
const ll LINF = 1001002003004005006ll;
int dx[] = { 1,0,-1,0 }, dy[] = { 0,1,0,-1 };
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; }return false; }
template<class T>bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; }return false; }

struct IOSetup {
    IOSetup() {
        cin.tie(0);
        ios::sync_with_stdio(0);
        cout << fixed << setprecision(12);
    }
} iosetup;

template<typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    for (int i = 0; i < (int)v.size(); i++) os << v[i] << (i + 1 == (int)v.size() ? "" : " ");
    return os;
}
template<typename T>
istream& operator>>(istream& is, vector<T>& v) {
    for (T& x : v)is >> x;
    return is;
}

#line 1 "Geometry/template.cpp"
// Real
using Real = double;
const Real EPS = 1e-13;
const Real pi = acosl(-1);

// Point
using Point = complex<Real>;
istream& operator>>(istream& is, Point& p) {
    Real a, b;
    is >> a >> b;
    p = Point(a, b);
    return is;
}
ostream& operator<<(ostream& os, Point& p) {
    return os << fixed << setprecision(12) << p.real() << ' ' << p.imag();
}
inline bool eq(Real a, Real b) {
    return fabs(a - b) < EPS;
}
Point operator*(const Point& p, const Real& d) {
    return Point(real(p) * d, imag(p) * d);
}

// Line
struct Line {
    Point p1, p2;
    Line() = default;
    Line(Point p1, Point p2) :p1(p1), p2(p2) {}
    //Ax + By = C
    Line(Real A, Real B, Real C) {
        if (eq(A, 0))     p1 = Point(0, C / B), p2 = Point(1, C / B);
        else if (eq(B, 0))p1 = Point(C / A, 0), p2 = Point(C / A, 1);
        else            p1 = Point(0, C / B), p2 = Point(C / A, 0);
    }
};

// Segment
struct Segment :Line {
    Segment() = default;
    Segment(Point p1, Point p2) :Line(p1, p2) {}
};
// Polygon
using Polygon = vector<Point>;
#line 1 "Geometry/Rotate.cpp"
Point rotate(Real theta, Point p) {
    return Point(cos(theta) * real(p) - sin(theta) * imag(p), sin(theta) * real(p) + cos(theta) * imag(p));
}
#line 1 "Geometry/Dot.cpp"
// Dot
Real dot(Point a, Point b) {
    return real(a) * real(b) + imag(a) * imag(b);
}
#line 1 "Geometry/Cross.cpp"
// Cross
Real cross(Point a, Point b) {
    return real(a) * imag(b) - imag(a) * real(b);
}

int contains(Polygon poly, Point p) {
    int res = 0;
    int n = (int)poly.size();
    for (int i = 0; i < n; i++) {
        Point a = poly[i] - p, b = poly[(i + 1) % n] - p;
        if (imag(a) > imag(b)) swap(a, b);
        if (imag(a) <= 0 && 0 < imag(b) && cross(a, b) < 0) res ^= 1;
        if (eq(cross(a, b), 0) && (dot(a, b) < 0 || eq(dot(a, b), 0))) return 1;
    }
    if (res) res = 2;
    return res;
}

vector<Point> convex_hull(vector<Point> p) {
    int n = (int)p.size(), k = 0;
    if (n <= 2)return p;
    sort(begin(p), end(p), [](Point a, Point b) {
        return real(a) != real(b) ? real(a) < real(b) : imag(a) < imag(b);
        });
    vector<Point>ch(2 * n);
    for (int i = 0; i < n; ch[k++] = p[i++]) {
        while (k >= 2 && cross(ch[k - 1] - ch[k - 2], p[i] - ch[k - 1]) < 0)k--;
    }
    for (int i = n - 2, t = k + 1; i >= 0; ch[k++] = p[i--]) {
        while (k >= t && cross(ch[k - 1] - ch[k - 2], p[i] - ch[k - 1]) < 0)k--;
    }
    ch.resize(k - 1);
    return ch;
}

int main() {
    ll N;
    cin>>N;
    vector<Point> P(N);
    for(int i=0;i<N;i++){
        cin>>P[i];
    }
    for(int i=0;i+1<N;i++){
        vector<Point> Q,R;
        for(int j=0;j<N;j++)(j<=i?Q:R).push_back(P[j]);
        if(contains(convex_hull(Q),P[i+1])||contains(convex_hull(R),P[i])){
            cout<<"Impossible"<<endl;
            return 0;
        }
    }
    cout<<"Possible"<<endl;
}
accuracy: 1
time_ms: 50
memory_kb: 3660
final_rank: 5

code_snippet: #include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i,n) for(ll i=0;i<ll(n);i++)
#define ALL(A) A.begin(),A.end()

#include<complex>
#define mod 1000000007
using ll = long long;
const int INF = 1000000000;
const ll LINF = 1001002003004005006ll;
int dx[] = { 1,0,-1,0 }, dy[] = { 0,1,0,-1 };
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
template<class T>bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; }return false; }
template<class T>bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; }return false; }

struct IOSetup {
    IOSetup() {
        cin.tie(0);
        ios::sync_with_stdio(0);
        cout << fixed << setprecision(12);
    }
} iosetup;

template<typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    for (int i = 0; i < (int)v.size(); i++) os << v[i] << (i + 1 == (int)v.size() ? "" : " ");
    return os;
}
template<typename T>
istream& operator>>(istream& is, vector<T>& v) {
    for (T& x : v)is >> x;
    return is;
}

#line 1 "Geometry/template.cpp"
// Real
using Real = double;
const Real EPS = 1e-13;
const Real pi = acosl(-1);

// Point
using Point = complex<Real>;
istream& operator>>(istream& is, Point& p) {
    Real a, b;
    is >> a >> b;
    p = Point(a, b);
    return is;
}
ostream& operator<<(ostream& os, Point& p) {
    return os << fixed << setprecision(12) << p.real() << ' ' << p.imag();
}
inline bool eq(Real a, Real b) {
    return fabs(a - b) < EPS;
}
Point operator*(const Point& p, const Real& d) {
    return Point(real(p) * d, imag(p) * d);
}

// Line
struct Line {
    Point p1, p2;
    Line() = default;
    Line(Point p1, Point p2) :p1(p1), p2(p2) {}
    //Ax + By = C
    Line(Real A, Real B, Real C) {
        if (eq(A, 0))     p1 = Point(0, C / B), p2 = Point(1, C / B);
        else if (eq(B, 0))p1 = Point(C / A, 0), p2 = Point(C / A, 1);
        else            p1 = Point(0, C / B), p2 = Point(C / A, 0);
    }
};

// Segment
struct Segment :Line {
    Segment() = default;
    Segment(Point p1, Point p2) :Line(p1, p2) {}
};
// Polygon
using Polygon = vector<Point>;
#line 1 "Geometry/Rotate.cpp"
Point rotate(Real theta, Point p) {
    return Point(cos(theta) * real(p) - sin(theta) * imag(p), sin(theta) * real(p) + cos(theta) * imag(p));
}
#line 1 "Geometry/Dot.cpp"
// Dot
Real dot(Point a, Point b) {
    return real(a) * real(b) + imag(a) * imag(b);
}
#line 1 "Geometry/Cross.cpp"
// Cross
Real cross(Point a, Point b) {
    return real(a) * imag(b) - imag(a) * real(b);
}

int contains(Polygon poly, Point p) {
    int res = 0;
    int n = (int)poly.size();
    for (int i = 0; i < n; i++) {
        Point a = poly[i] - p, b = poly[(i + 1) % n] - p;
        if (imag(a) > imag(b)) swap(a, b);
        if (imag(a) <= 0 && 0 < imag(b) && cross(a, b) < 0) res ^= 1;
        if (eq(cross(a, b), 0) && (dot(a, b) < 0 || eq(dot(a, b), 0))) return 1;
    }
    if (res) res = 2;
    return res;
}

vector<Point> convex_hull(vector<Point> p) {
    int n = (int)p.size(), k = 0;
    if (n <= 2)return p;
    sort(begin(p), end(p), [](Point a, Point b) {
        return real(a) != real(b) ? real(a) < real(b) : imag(a) < imag(b);
        });
    vector<Point>ch(2 * n);
    for (int i = 0; i < n; ch[k++] = p[i++]) {
        while (k >= 2 && cross(ch[k - 1] - ch[k - 2], p[i] - ch[k - 1]) < 0)k--;
    }
    for (int i = n - 2, t = k + 1; i >= 0; ch[k++] = p[i--]) {
        while (k >= t && cross(ch[k - 1] - ch[k - 2], p[i] - ch[k - 1]) < 0)k--;
    }
    ch.resize(k - 1);
    return ch;
}

int main() {
    ll N;
    cin>>N;
    vector<Point> P(N);
    for(int i=0;i<N;i++){
        cin>>P[i];
    }
    for(int i=1;i+2<N;i++){
        vector<Point> Q,R;
        for(int j=0;j<N;j++)(j<=i?Q:R).push_back(P[j]);
        if(contains(convex_hull(Q),P[i+1])||contains(convex_hull(R),P[i])){
            cout<<"Impossible"<<endl;
            return 0;
        }
    }
    cout<<"Possible"<<endl;
}
accuracy: 0.007352941176470588
time_ms: 30
memory_kb: 3668
final_rank: 20

code_snippet: #include <bits/stdc++.h>
using namespace std;

// #pragma GCC target("avx2")
// #pragma GCC optimize("Ofast")
// #pragma GCC optimize("unroll-loops")

// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")

using ll = long long;
using Real = long double;
using Point = complex<Real>;
const Real EPS = 1e-9, PI = acos(-1);

inline bool eq(Real a, Real b) {
  return abs(b - a) < EPS;
}

using Points = vector<Point>;
using Polygon = vector<Point>;

namespace std {
bool operator<(const Point &a, const Point &b) {
  return a.real() != b.real() ? a.real() < b.real() : a.imag() < b.imag();
}
}

Real cross(const Point &a, const Point &b) {
  return real(a) * imag(b) - imag(a) * real(b);
}

Real dot(const Point &a, const Point &b) {
  return real(a) * real(b) + imag(a) * imag(b);
}

// Polygon convex_hull(Polygon &p) {
//   int n = (int)p.size(), k = 0;
//   if (n <= 2) return p;
//   sort(p.begin(), p.end());
//   vector<Point> ch(2 * n);
//   for (int i = 0; i < n; ch[k++] = p[i++]) {
//     while (k >= 2 && cross(ch[k-1] - ch[k-2], p[i] - ch[k-1]) < EPS) --k;
//   }
//   for (int i = n-2, t = k+1; i >= 0; ch[k++] = p[i--]) {
//     while (k >= t && cross(ch[k-1] - ch[k-2], p[i] - ch[k-1]) < EPS) --k;
//   }
//   ch.resize(k - 1);
//   return ch;
// }

inline Real cross(Point &a, Point &b, Point &c) {
  return ( ((b.real() - a.real()) * (c.imag() - a.imag())) \
    - ((b.imag() - a.imag()) * (c.real() - a.real())) );
}

void convex_hull(vector<Point> &ps, vector<Point> &qs) {
  // sort(ps.begin(), ps.end());
  qs.clear();
  qs.reserve(ps.size());
  int n = ps.size();
  for(auto p : ps) {
    // 外積判定の等号なし => 凸包の直線上にある点も含む
    while(qs.size() > 1 && cross(qs[qs.size()-2], qs[qs.size()-1], p) > 0) {
      qs.pop_back();
    }
    qs.push_back(p);
  }
  int t = qs.size();
 
  for(int i = n-2; i >= 0; --i) {
    Point &p = ps[i];
    while(qs.size() > t && cross(qs[qs.size()-2], qs[qs.size()-1], p) > 0) {
      qs.pop_back();
    }
    qs.push_back(p);
  }
  qs.pop_back();
  
  return;
}


enum { OUT, ON, IN };

int contains(const Polygon &Q, const Point &p) {
  bool in = false;
  for (int i = 0; i < Q.size(); ++i) {
    Point a = Q[i] - p, b = Q[(i+1) % Q.size()] - p;
    if (a.imag() > b.imag()) swap(a, b);
    if (a.imag() <= 0 && 0 < b.imag() && cross(a, b) < 0) in = !in;
    if (cross(a, b) == 0 && dot(a, b) <= 0) return ON;
  }
  return in ? IN : OUT;
}

int main() {
  int N; cin >> N;
  vector<Point> pos(N);
  for (int i = 0; i < N; ++i) {
    Real x, y; cin >> x >> y;
    pos[i] = Point(x, y);
  }
  
  bool res = true;
  for (int i = 0; i + 1 < N; ++i) {
    Polygon lower(pos.begin(), pos.begin() + i + 1);
    Polygon low_hull;
    convex_hull(lower, low_hull);
    
    Polygon upper(pos.begin() + i + 1, pos.end());
    Polygon up_hull;
    convex_hull(upper, up_hull);
    
    // cout << i << endl;
    // for (auto &c : up_hull)
    // {
    //   cout << c.real() << " " << c.imag() << endl;
    // }
    
    bool tmp = true;
    if (contains(low_hull, pos[i+1]) != OUT) {
      tmp = false;
    }
    if (contains(up_hull, pos[i]) != OUT) {
      tmp = false;
    }
    res &= tmp;
    
  }
  
  cout << (res ? "Possible" : "Impossible") << endl;
  
  return 0;
}
accuracy: 1
time_ms: 30
memory_kb: 3696
final_rank: 7

code_snippet: #include <bits/stdc++.h>
using namespace std;

// #pragma GCC target("avx2")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")

#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")

using ll = long long;
using Real = long double;
using Point = complex<Real>;
const Real EPS = 1e-9, PI = acos(-1);

inline bool eq(Real a, Real b) {
  return abs(b - a) < EPS;
}

using Points = vector<Point>;
using Polygon = vector<Point>;

namespace std {
bool operator<(const Point &a, const Point &b) {
  return a.real() != b.real() ? a.real() < b.real() : a.imag() < b.imag();
}
}

Real cross(const Point &a, const Point &b) {
  return real(a) * imag(b) - imag(a) * real(b);
}

Real dot(const Point &a, const Point &b) {
  return real(a) * real(b) + imag(a) * imag(b);
}

Polygon convex_hull(Polygon &p) {
  int n = (int)p.size(), k = 0;
  if (n <= 2) return p;
  sort(p.begin(), p.end());
  vector<Point> ch(2 * n);
  for (int i = 0; i < n; ch[k++] = p[i++]) {
    while (k >= 2 && cross(ch[k-1] - ch[k-2], p[i] - ch[k-1]) < EPS) --k;
  }
  for (int i = n-2, t = k+1; i >= 0; ch[k++] = p[i--]) {
    while (k >= t && cross(ch[k-1] - ch[k-2], p[i] - ch[k-1]) < EPS) --k;
  }
  ch.resize(k - 1);
  return ch;
}

enum { OUT, ON, IN };

int contains(const Polygon &Q, const Point &p) {
  bool in = false;
  for (int i = 0; i < Q.size(); ++i) {
    Point a = Q[i] - p, b = Q[(i+1) % Q.size()] - p;
    if (a.imag() > b.imag()) swap(a, b);
    if (a.imag() <= 0 && 0 < b.imag() && cross(a, b) < 0) in = !in;
    if (cross(a, b) == 0 && dot(a, b) <= 0) return ON;
  }
  return in ? IN : OUT;
}

int main() {
  int N; cin >> N;
  vector<Point> pos(N);
  for (int i = 0; i < N; ++i) {
    Real x, y; cin >> x >> y;
    pos[i] = Point(x, y);
  }
  
  bool res = true;
  for (int i = 0; i + 1 < N; ++i) {
    Polygon lower(pos.begin(), pos.begin() + i + 1);
    convex_hull(lower);
    Polygon upper(pos.begin() + i + 1, pos.end());
    convex_hull(upper);
    
    bool tmp = true;
    if (i+1 < N && contains(lower, pos[i+1]) != OUT) {
      tmp = false;
    }
    if (contains(upper, pos[i]) != OUT) {
      tmp = false;
    }
    res &= tmp;
    
  }
  
  cout << (res ? "Possible" : "Impossible") << endl;
  
  return 0;
}
accuracy: 0.27205882352941174
time_ms: 110
memory_kb: 3660
final_rank: 10

code_snippet: #include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, t) for (int i = (int)(s); i < (int)(t); ++i)
#define revrep(i, t, s) for (int i = (int)(t)-1; i >= (int)(s); --i)
#define all(x) begin(x), end(x)
template <typename T> bool chmax(T& a, const T& b) { return a < b ? (a = b, 1) : 0; }
template <typename T> bool chmin(T& a, const T& b) { return a > b ? (a = b, 1) : 0; }

using T = ll;
using Vec = std::complex<T>;

const T PI = std::acos(-1);

constexpr T eps = 1e-10;
inline bool eq(T a, T b) { return std::abs(a - b) <= eps; }
inline bool eq(Vec a, Vec b) { return std::abs(a - b) <= eps; }
inline bool lt(T a, T b) { return a < b - eps; }
inline bool leq(T a, T b) { return a <= b + eps; }

std::istream& operator>>(std::istream& is, Vec& p) {
    T x, y;
    is >> x >> y;
    p = {x, y};
    return is;
}

struct Line {
    Vec p1, p2;
    Line() = default;
    Line(const Vec& p1, const Vec& p2) : p1(p1), p2(p2) {}
    Vec dir() const { return p2 - p1; }
};

struct Segment {
    Vec p1, p2;
    Segment() = default;
    Segment(const Vec& p1, const Vec& p2) : p1(p1), p2(p2) {}
    Vec dir() const { return p2 - p1; }
};

struct Circle {
    Vec c;
    T r;
    Circle() = default;
    Circle(const Vec& c, T r) : c(c), r(r) {}
};

using Polygon = std::vector<Vec>;

T dot(const Vec& a, const Vec& b) {
    return (std::conj(a) * b).real();
}

T cross(const Vec& a, const Vec& b) {
    return (std::conj(a) * b).imag();
}

Vec rot(const Vec& a, T ang) {
    return a * Vec(std::cos(ang), std::sin(ang));
}

Vec perp(const Vec& a) {
    return Vec(-a.imag(), a.real());
}

Vec projection(const Line& l, const Vec& p) {
    return l.p1 + dot(p - l.p1, l.dir()) * l.dir() / std::norm(l.dir());
}

Vec reflection(const Line& l, const Vec& p) {
    return T(2) * projection(l, p) - p;
}

// 0: collinear
// 1: counter-clockwise
// 2: clockwise
int ccw(const Vec& a, const Vec& b, const Vec& c) {
    if (eq(cross(b - a, c - a), 0)) return 0;
    if (lt(cross(b - a, c - a), 0)) return -1;
    return 1;
}

void sort_by_arg(std::vector<Vec>& pts) {
    std::sort(pts.begin(), pts.end(), [&](auto& p, auto& q) {
        if ((p.imag() < 0) != (q.imag() < 0)) return (p.imag() < 0);
        if (cross(p, q) == 0) {
            if (p == Vec(0, 0)) return !(q.imag() < 0 || (q.imag() == 0 && q.real() > 0));
            if (q == Vec(0, 0)) return  (p.imag() < 0 || (p.imag() == 0 && p.real() > 0));
            return (p.real() > q.real());
        }
        return (cross(p, q) > 0);
    });
}

bool intersect(const Segment& s, const Vec& p) {
    Vec u = s.p1 - p, v = s.p2 - p;
    return eq(cross(u, v), 0) && leq(dot(u, v), 0);
}

// 0: outside
// 1: on the border
// 2: inside
int intersect(const Polygon& poly, const Vec& p) {
    const int n = poly.size();
    bool in = 0;
    for (int i = 0; i < n; ++i) {
        auto a = poly[i] - p, b = poly[(i+1)%n] - p;
        if (eq(cross(a, b), 0) && (lt(dot(a, b), 0) || eq(dot(a, b), 0))) return 1;
        if (a.imag() > b.imag()) std::swap(a, b);
        if (leq(a.imag(), 0) && lt(0, b.imag()) && lt(cross(a, b), 0)) in ^= 1;
    }
    return in ? 2 : 0;
}


int intersect(const Segment& s, const Segment& t) {
    auto a = s.p1, b = s.p2;
    auto c = t.p1, d = t.p2;
    if (ccw(a, b, c) != ccw(a, b, d) && ccw(c, d, a) != ccw(c, d, b)) return 2;
    if (intersect(s, c) || intersect(s, d) || intersect(t, a) || intersect(t, b)) return 1;
    return 0;
}

// true if they have positive area in common or touch on the border
bool intersect(const Polygon& poly1, const Polygon& poly2) {
    const int n = poly1.size();
    const int m = poly2.size();
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < m; ++j) {
            if (intersect(Segment(poly1[i], poly1[(i+1)%n]), Segment(poly2[j], poly2[(j+1)%m]))) {
                return true;
            }
        }
    }
    return intersect(poly1, poly2[0]) || intersect(poly2, poly1[0]);
}

bool intersect(const Polygon& poly, const Line& l) {
    int side = 0;
    for (auto& p : poly) {
        int s = ccw(l.p1, l.p2, p);
        if (s == 0) continue;
        if (side == 0) side = s;
        else if (side != s) return true;
    }
    return false;
}

std::vector<Vec> convex_hull(std::vector<Vec>& pts) {
    int n = pts.size();
    if (n == 1) return pts;
    std::sort(pts.begin(), pts.end(), [](const Vec& v1, const Vec& v2) {
        return (v1.imag() != v2.imag()) ? (v1.imag() < v2.imag()) : (v1.real() < v2.real());
    });
    int k = 0; // the number of vertices in the convex hull
    std::vector<Vec> ch(2 * n);
    // right
    for (int i = 0; i < n; ++i) {
        while (k > 1 && leq(cross(ch[k-1] - ch[k-2], pts[i] - ch[k-1]), 0)) --k;
        ch[k++] = pts[i];
    }
    int t = k;
    // left
    for (int i = n - 2; i >= 0; --i) {
        while (k > t && leq(cross(ch[k-1] - ch[k-2], pts[i] - ch[k-1]), 0)) --k;
        ch[k++] = pts[i];
    }
    ch.resize(k - 1);
    return ch;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(15);

    int n;
    cin >> n;
    vector<Vec> pts(n);
    for (auto& x : pts) cin >> x;
    rep(i,0,n-1) {
        vector<Vec> first, second;
        rep(j,0,n) {
            if (j <= i) first.push_back(pts[j]);
            else second.push_back(pts[j]);
        }
        first = convex_hull(first);
        second = convex_hull(second);
        if (intersect(first, second)) {
            cout << "Impossible" << endl;
            return 0;
        }
    }
    cout << "Possible" << endl;
}
accuracy: 1
time_ms: 1260
memory_kb: 3552
final_rank: 8

code_snippet: #include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, t) for (int i = (int)(s); i < (int)(t); ++i)
#define revrep(i, t, s) for (int i = (int)(t)-1; i >= (int)(s); --i)
#define all(x) begin(x), end(x)
template <typename T> bool chmax(T& a, const T& b) { return a < b ? (a = b, 1) : 0; }
template <typename T> bool chmin(T& a, const T& b) { return a > b ? (a = b, 1) : 0; }

using T = ll;
using Vec = std::complex<T>;

const T PI = std::acos(-1);

constexpr T eps = 1e-10;
inline bool eq(T a, T b) { return std::abs(a - b) <= eps; }
inline bool eq(Vec a, Vec b) { return std::abs(a - b) <= eps; }
inline bool lt(T a, T b) { return a < b - eps; }
inline bool leq(T a, T b) { return a <= b + eps; }

std::istream& operator>>(std::istream& is, Vec& p) {
    T x, y;
    is >> x >> y;
    p = {x, y};
    return is;
}

struct Line {
    Vec p1, p2;
    Line() = default;
    Line(const Vec& p1, const Vec& p2) : p1(p1), p2(p2) {}
    Vec dir() const { return p2 - p1; }
};

struct Segment {
    Vec p1, p2;
    Segment() = default;
    Segment(const Vec& p1, const Vec& p2) : p1(p1), p2(p2) {}
    Vec dir() const { return p2 - p1; }
};

struct Circle {
    Vec c;
    T r;
    Circle() = default;
    Circle(const Vec& c, T r) : c(c), r(r) {}
};

using Polygon = std::vector<Vec>;

T dot(const Vec& a, const Vec& b) {
    return (std::conj(a) * b).real();
}

T cross(const Vec& a, const Vec& b) {
    return (std::conj(a) * b).imag();
}

Vec rot(const Vec& a, T ang) {
    return a * Vec(std::cos(ang), std::sin(ang));
}

Vec perp(const Vec& a) {
    return Vec(-a.imag(), a.real());
}

Vec projection(const Line& l, const Vec& p) {
    return l.p1 + dot(p - l.p1, l.dir()) * l.dir() / std::norm(l.dir());
}

Vec reflection(const Line& l, const Vec& p) {
    return T(2) * projection(l, p) - p;
}

// 0: collinear
// 1: counter-clockwise
// 2: clockwise
int ccw(const Vec& a, const Vec& b, const Vec& c) {
    if (eq(cross(b - a, c - a), 0)) return 0;
    if (lt(cross(b - a, c - a), 0)) return -1;
    return 1;
}

void sort_by_arg(std::vector<Vec>& pts) {
    std::sort(pts.begin(), pts.end(), [&](auto& p, auto& q) {
        if ((p.imag() < 0) != (q.imag() < 0)) return (p.imag() < 0);
        if (cross(p, q) == 0) {
            if (p == Vec(0, 0)) return !(q.imag() < 0 || (q.imag() == 0 && q.real() > 0));
            if (q == Vec(0, 0)) return  (p.imag() < 0 || (p.imag() == 0 && p.real() > 0));
            return (p.real() > q.real());
        }
        return (cross(p, q) > 0);
    });
}

bool intersect(const Segment& s, const Vec& p) {
    Vec u = s.p1 - p, v = s.p2 - p;
    return eq(cross(u, v), 0) && leq(dot(u, v), 0);
}

// 0: outside
// 1: on the border
// 2: inside
int intersect(const Polygon& poly, const Vec& p) {
    const int n = poly.size();
    bool in = 0;
    for (int i = 0; i < n; ++i) {
        auto a = poly[i] - p, b = poly[(i+1)%n] - p;
        if (eq(cross(a, b), 0) && (lt(dot(a, b), 0) || eq(dot(a, b), 0))) return 1;
        if (a.imag() > b.imag()) std::swap(a, b);
        if (leq(a.imag(), 0) && lt(0, b.imag()) && lt(cross(a, b), 0)) in ^= 1;
    }
    return in ? 2 : 0;
}


int intersect(const Segment& s, const Segment& t) {
    auto a = s.p1, b = s.p2;
    auto c = t.p1, d = t.p2;
    if (ccw(a, b, c) != ccw(a, b, d) && ccw(c, d, a) != ccw(c, d, b)) return 2;
    if (intersect(s, c) || intersect(s, d) || intersect(t, a) || intersect(t, b)) return 1;
    return 0;
}

// true if they have positive area in common or touch on the border
bool intersect(const Polygon& poly1, const Polygon& poly2) {
    const int n = poly1.size();
    const int m = poly2.size();
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < m; ++j) {
            if (intersect(Segment(poly1[i], poly1[(i+1)%n]), Segment(poly2[j], poly2[(j+1)%m]))) {
                return true;
            }
        }
    }
    return intersect(poly1, poly2[0]) || intersect(poly2, poly1[0]);
}

bool intersect(const Polygon& poly, const Line& l) {
    int side = 0;
    for (auto& p : poly) {
        int s = ccw(l.p1, l.p2, p);
        if (s == 0) continue;
        if (side == 0) side = s;
        else if (side != s) return true;
    }
    return false;
}

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    for (int i = 0; i < (int) v.size(); ++i) os << v[i] << " ";
    return os;
}


bool separate(const Polygon& poly1, const Polygon& poly2, const Line& l) {
    int side1 = 0;
    for (auto& p : poly1) {
        int s = ccw(l.p1, l.p2, p);
        if (s == 0) continue;
        if (side1 == 0) side1 = s;
        else if (side1 != s) return false;
    }
    int side2 = 0;
    for (auto& p : poly2) {
        int s = ccw(l.p1, l.p2, p);
        if (s == 0) continue;
        if (side2 == 0) side2 = s;
        else if (side2 != s) return false;
    }
    return side1 * side2 <= 0;
}


std::vector<Vec> convex_hull(std::vector<Vec>& pts) {
    int n = pts.size();
    if (n == 1) return pts;
    std::sort(pts.begin(), pts.end(), [](const Vec& v1, const Vec& v2) {
        return (v1.imag() != v2.imag()) ? (v1.imag() < v2.imag()) : (v1.real() < v2.real());
    });
    int k = 0; // the number of vertices in the convex hull
    std::vector<Vec> ch(2 * n);
    // right
    for (int i = 0; i < n; ++i) {
        while (k > 1 && leq(cross(ch[k-1] - ch[k-2], pts[i] - ch[k-1]), 0)) --k;
        ch[k++] = pts[i];
    }
    int t = k;
    // left
    for (int i = n - 2; i >= 0; --i) {
        while (k > t && leq(cross(ch[k-1] - ch[k-2], pts[i] - ch[k-1]), 0)) --k;
        ch[k++] = pts[i];
    }
    ch.resize(k - 1);
    return ch;
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(15);

    int n;
    cin >> n;
    vector<Vec> pts(n);
    for (auto& x : pts) cin >> x;
    rep(i,0,n-1) {
        vector<Vec> first, second;
        rep(j,0,n) {
            if (j <= i) first.push_back(pts[j]);
            else second.push_back(pts[j]);
        }
        first = convex_hull(first);
        second = convex_hull(second);
        // cout << first << endl;
        // cout << second << endl;
        if (intersect(first, second)) {
            cout << "Impossible" << endl;
            return 0;
        }
        // int m = first.size();
        // if (m > 1) {
        //     bool ok = false;
        //     rep(j,0,m) {
        //         if (first[j] == pts[i]) {
        //             if (!separate(first, second, Line(first[(j-1+m)%m], first[j])) &&
        //                 !separate(first, second, Line(first[j], first[(j+1)%m]))) {

        //                 cout << "Impossible" << endl;
        //                 return 0;
        //             }
        //             ok = true;
        //         }
        //     }
        //     if (!ok) {
        //         cout << "Impossible" << endl;
        //         return 0;
        //     }
        // }
        // m = second.size();
        // if (m > 1) {
        //     bool ok = false;
        //     rep(j,0,m) {
        //         if (second[j] == pts[i+1]) {
        //             if (!separate(first, second, Line(second[(j-1+m)%m], second[j])) &&
        //                 !separate(first, second, Line(second[j], second[(j+1)%m]))) {

        //                 cout << "Impossible" << endl;
        //                 return 0;
        //             }
        //             ok = true;
        //         }
        //     }
        //     if (!ok) {
        //         cout << "Impossible" << endl;
        //         return 0;
        //     }
        // }
    }
    cout << "Possible" << endl;
}
accuracy: 1
time_ms: 1260
memory_kb: 3556
final_rank: 9

code_snippet: #include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, t) for (int i = (int)(s); i < (int)(t); ++i)
#define revrep(i, t, s) for (int i = (int)(t)-1; i >= (int)(s); --i)
#define all(x) begin(x), end(x)
template <typename T> bool chmax(T& a, const T& b) { return a < b ? (a = b, 1) : 0; }
template <typename T> bool chmin(T& a, const T& b) { return a > b ? (a = b, 1) : 0; }

using T = ll;
using Vec = std::complex<T>;

const T PI = std::acos(-1);

constexpr T eps = 1e-12;
inline bool eq(T a, T b) { return std::abs(a - b) <= eps; }
inline bool eq(Vec a, Vec b) { return std::abs(a - b) <= eps; }
inline bool lt(T a, T b) { return a < b - eps; }
inline bool leq(T a, T b) { return a <= b + eps; }

std::istream& operator>>(std::istream& is, Vec& p) {
    T x, y;
    is >> x >> y;
    p = {x, y};
    return is;
}

struct Line {
    Vec p1, p2;
    Line() = default;
    Line(const Vec& p1, const Vec& p2) : p1(p1), p2(p2) {}
    Vec dir() const { return p2 - p1; }
};

struct Segment {
    Vec p1, p2;
    Segment() = default;
    Segment(const Vec& p1, const Vec& p2) : p1(p1), p2(p2) {}
    Vec dir() const { return p2 - p1; }
};

struct Circle {
    Vec c;
    T r;
    Circle() = default;
    Circle(const Vec& c, T r) : c(c), r(r) {}
};

using Polygon = std::vector<Vec>;

T dot(const Vec& a, const Vec& b) {
    return (std::conj(a) * b).real();
}

T cross(const Vec& a, const Vec& b) {
    return (std::conj(a) * b).imag();
}

Vec rot(const Vec& a, T ang) {
    return a * Vec(std::cos(ang), std::sin(ang));
}

Vec perp(const Vec& a) {
    return Vec(-a.imag(), a.real());
}

Vec projection(const Line& l, const Vec& p) {
    return l.p1 + dot(p - l.p1, l.dir()) * l.dir() / std::norm(l.dir());
}

Vec reflection(const Line& l, const Vec& p) {
    return T(2) * projection(l, p) - p;
}

// 0: collinear
// 1: counter-clockwise
// 2: clockwise
int ccw(const Vec& a, const Vec& b, const Vec& c) {
    if (eq(cross(b - a, c - a), 0)) return 0;
    if (lt(cross(b - a, c - a), 0)) return -1;
    return 1;
}

void sort_by_arg(std::vector<Vec>& pts) {
    std::sort(pts.begin(), pts.end(), [&](auto& p, auto& q) {
        if ((p.imag() < 0) != (q.imag() < 0)) return (p.imag() < 0);
        if (cross(p, q) == 0) {
            if (p == Vec(0, 0)) return !(q.imag() < 0 || (q.imag() == 0 && q.real() > 0));
            if (q == Vec(0, 0)) return  (p.imag() < 0 || (p.imag() == 0 && p.real() > 0));
            return (p.real() > q.real());
        }
        return (cross(p, q) > 0);
    });
}

bool intersect(const Segment& s, const Vec& p) {
    Vec u = s.p1 - p, v = s.p2 - p;
    return eq(cross(u, v), 0) && leq(dot(u, v), 0);
}

// 0: outside
// 1: on the border
// 2: inside
int intersect(const Polygon& poly, const Vec& p) {
    const int n = poly.size();
    bool in = 0;
    for (int i = 0; i < n; ++i) {
        auto a = poly[i] - p, b = poly[(i+1)%n] - p;
        if (eq(cross(a, b), 0) && (lt(dot(a, b), 0) || eq(dot(a, b), 0))) return 1;
        if (a.imag() > b.imag()) std::swap(a, b);
        if (leq(a.imag(), 0) && lt(0, b.imag()) && lt(cross(a, b), 0)) in ^= 1;
    }
    return in ? 2 : 0;
}


int intersect(const Segment& s, const Segment& t) {
    auto a = s.p1, b = s.p2;
    auto c = t.p1, d = t.p2;
    if (ccw(a, b, c) != ccw(a, b, d) && ccw(c, d, a) != ccw(c, d, b)) return 2;
    if (intersect(s, c) || intersect(s, d) || intersect(t, a) || intersect(t, b)) return 1;
    return 0;
}

// true if they have positive area in common or touch on the border
bool intersect(const Polygon& poly1, const Polygon& poly2) {
    const int n = poly1.size();
    const int m = poly2.size();
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < m; ++j) {
            if (intersect(Segment(poly1[i], poly1[(i+1)%n]), Segment(poly2[j], poly2[(j+1)%m]))) {
                return true;
            }
        }
    }
    return intersect(poly1, poly2[0]) || intersect(poly2, poly1[0]);
}

bool intersect(const Polygon& poly, const Line& l) {
    int side = 0;
    for (auto& p : poly) {
        int s = ccw(l.p1, l.p2, p);
        if (s == 0) continue;
        if (side == 0) side = s;
        else if (side != s) return true;
    }
    return false;
}

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    for (int i = 0; i < (int) v.size(); ++i) os << v[i] << " ";
    return os;
}


bool separate(const Polygon& poly1, const Polygon& poly2, const Line& l) {
    int side1 = 0;
    for (auto& p : poly1) {
        int s = ccw(l.p1, l.p2, p);
        if (s == 0) continue;
        if (side1 == 0) side1 = s;
        else if (side1 != s) return false;
    }
    int side2 = 0;
    for (auto& p : poly2) {
        int s = ccw(l.p1, l.p2, p);
        if (s == 0) continue;
        if (side2 == 0) side2 = s;
        else if (side2 != s) return false;
    }
    return side1 * side2 <= 0;
}


std::vector<Vec> convex_hull(std::vector<Vec>& pts) {
    int n = pts.size();
    if (n == 1) return pts;
    std::sort(pts.begin(), pts.end(), [](const Vec& v1, const Vec& v2) {
        return (v1.imag() != v2.imag()) ? (v1.imag() < v2.imag()) : (v1.real() < v2.real());
    });
    int k = 0; // the number of vertices in the convex hull
    std::vector<Vec> ch(2 * n);
    // right
    for (int i = 0; i < n; ++i) {
        while (k > 1 && leq(cross(ch[k-1] - ch[k-2], pts[i] - ch[k-1]), 0)) --k;
        ch[k++] = pts[i];
    }
    int t = k;
    // left
    for (int i = n - 2; i >= 0; --i) {
        while (k > t && leq(cross(ch[k-1] - ch[k-2], pts[i] - ch[k-1]), 0)) --k;
        ch[k++] = pts[i];
    }
    ch.resize(k - 1);
    return ch;
}


int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(15);

    int n;
    cin >> n;
    vector<Vec> pts(n);
    for (auto& x : pts) cin >> x;
    rep(i,0,n-1) {
        vector<Vec> first, second;
        rep(j,0,n) {
            if (j <= i) first.push_back(pts[j]);
            else second.push_back(pts[j]);
        }
        first = convex_hull(first);
        second = convex_hull(second);
        // if (intersect(first, second)) {
        //     cout << "Impossible" << endl;
        //     return 0;
        // }
        int m = first.size();
        bool ok = false;
        rep(j,0,m) {
            if (first[j] == pts[i]) {
                if (!separate(first, second, Line(first[(j-1+m)%m], first[j])) &&
                    !separate(first, second, Line(first[j], first[(j+1)%m]))) {

                    cout << "Impossible" << endl;
                    return 0;
                }
                ok = true;
            }
        }
        if (!ok) {
            cout << "Impossible" << endl;
            return 0;
        }
        m = second.size();
        ok = false;
        rep(j,0,m) {
            if (second[j] == pts[i+1]) {
                if (!separate(first, second, Line(second[(j-1+m)%m], second[j])) &&
                    !separate(first, second, Line(second[j], second[(j+1)%m]))) {

                    cout << "Impossible" << endl;
                    return 0;
                }
                ok = true;
            }
        }
        if (!ok) {
            cout << "Impossible" << endl;
            return 0;
        }
    }
    cout << "Possible" << endl;
}
accuracy: 0.014705882352941176
time_ms: 20
memory_kb: 3372
final_rank: 13

code_snippet: #include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, t) for (int i = (int)(s); i < (int)(t); ++i)
#define revrep(i, t, s) for (int i = (int)(t)-1; i >= (int)(s); --i)
#define all(x) begin(x), end(x)
template <typename T> bool chmax(T& a, const T& b) { return a < b ? (a = b, 1) : 0; }
template <typename T> bool chmin(T& a, const T& b) { return a > b ? (a = b, 1) : 0; }

using T = double;
using Vec = std::complex<T>;

const T PI = std::acos(-1);

constexpr T eps = 1e-12;
inline bool eq(T a, T b) { return std::abs(a - b) <= eps; }
inline bool eq(Vec a, Vec b) { return std::abs(a - b) <= eps; }
inline bool lt(T a, T b) { return a < b - eps; }
inline bool leq(T a, T b) { return a <= b + eps; }

std::istream& operator>>(std::istream& is, Vec& p) {
    T x, y;
    is >> x >> y;
    p = {x, y};
    return is;
}

struct Line {
    Vec p1, p2;
    Line() = default;
    Line(const Vec& p1, const Vec& p2) : p1(p1), p2(p2) {}
    Vec dir() const { return p2 - p1; }
};

struct Segment {
    Vec p1, p2;
    Segment() = default;
    Segment(const Vec& p1, const Vec& p2) : p1(p1), p2(p2) {}
    Vec dir() const { return p2 - p1; }
};

struct Circle {
    Vec c;
    T r;
    Circle() = default;
    Circle(const Vec& c, T r) : c(c), r(r) {}
};

using Polygon = std::vector<Vec>;

T dot(const Vec& a, const Vec& b) {
    return (std::conj(a) * b).real();
}

T cross(const Vec& a, const Vec& b) {
    return (std::conj(a) * b).imag();
}

Vec rot(const Vec& a, T ang) {
    return a * Vec(std::cos(ang), std::sin(ang));
}

Vec perp(const Vec& a) {
    return Vec(-a.imag(), a.real());
}

Vec projection(const Line& l, const Vec& p) {
    return l.p1 + dot(p - l.p1, l.dir()) * l.dir() / std::norm(l.dir());
}

Vec reflection(const Line& l, const Vec& p) {
    return T(2) * projection(l, p) - p;
}

// 0: collinear
// 1: counter-clockwise
// 2: clockwise
int ccw(const Vec& a, const Vec& b, const Vec& c) {
    if (eq(cross(b - a, c - a), 0)) return 0;
    if (lt(cross(b - a, c - a), 0)) return -1;
    return 1;
}

void sort_by_arg(std::vector<Vec>& pts) {
    std::sort(pts.begin(), pts.end(), [&](auto& p, auto& q) {
        if ((p.imag() < 0) != (q.imag() < 0)) return (p.imag() < 0);
        if (cross(p, q) == 0) {
            if (p == Vec(0, 0)) return !(q.imag() < 0 || (q.imag() == 0 && q.real() > 0));
            if (q == Vec(0, 0)) return  (p.imag() < 0 || (p.imag() == 0 && p.real() > 0));
            return (p.real() > q.real());
        }
        return (cross(p, q) > 0);
    });
}

bool intersect(const Segment& s, const Vec& p) {
    Vec u = s.p1 - p, v = s.p2 - p;
    return eq(cross(u, v), 0) && leq(dot(u, v), 0);
}

// 0: outside
// 1: on the border
// 2: inside
int intersect(const Polygon& poly, const Vec& p) {
    const int n = poly.size();
    bool in = 0;
    for (int i = 0; i < n; ++i) {
        auto a = poly[i] - p, b = poly[(i+1)%n] - p;
        if (eq(cross(a, b), 0) && (lt(dot(a, b), 0) || eq(dot(a, b), 0))) return 1;
        if (a.imag() > b.imag()) std::swap(a, b);
        if (leq(a.imag(), 0) && lt(0, b.imag()) && lt(cross(a, b), 0)) in ^= 1;
    }
    return in ? 2 : 0;
}


int intersect(const Segment& s, const Segment& t) {
    auto a = s.p1, b = s.p2;
    auto c = t.p1, d = t.p2;
    if (ccw(a, b, c) != ccw(a, b, d) && ccw(c, d, a) != ccw(c, d, b)) return 2;
    if (intersect(s, c) || intersect(s, d) || intersect(t, a) || intersect(t, b)) return 1;
    return 0;
}

// true if they have positive area in common or touch on the border
bool intersect(const Polygon& poly1, const Polygon& poly2) {
    const int n = poly1.size();
    const int m = poly2.size();
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < m; ++j) {
            if (intersect(Segment(poly1[i], poly1[(i+1)%n]), Segment(poly2[j], poly2[(j+1)%m]))) {
                return true;
            }
        }
    }
    return intersect(poly1, poly2[0]) || intersect(poly2, poly1[0]);
}

bool intersect(const Polygon& poly, const Line& l) {
    int side = 0;
    for (auto& p : poly) {
        int s = ccw(l.p1, l.p2, p);
        if (s == 0) continue;
        if (side == 0) side = s;
        else if (side != s) return true;
    }
    return false;
}

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v) {
    for (int i = 0; i < (int) v.size(); ++i) os << v[i] << " ";
    return os;
}


bool separate(const Polygon& poly1, const Polygon& poly2, const Line& l) {
    int side1 = 0;
    for (auto& p : poly1) {
        int s = ccw(l.p1, l.p2, p);
        if (s == 0) continue;
        if (side1 == 0) side1 = s;
        else if (side1 != s) return false;
    }
    int side2 = 0;
    for (auto& p : poly2) {
        int s = ccw(l.p1, l.p2, p);
        if (s == 0) continue;
        if (side2 == 0) side2 = s;
        else if (side2 != s) return false;
    }
    return side1 * side2 <= 0;
}


std::vector<Vec> convex_hull(std::vector<Vec>& pts) {
    int n = pts.size();
    if (n == 1) return pts;
    std::sort(pts.begin(), pts.end(), [](const Vec& v1, const Vec& v2) {
        return (v1.imag() != v2.imag()) ? (v1.imag() < v2.imag()) : (v1.real() < v2.real());
    });
    int k = 0; // the number of vertices in the convex hull
    std::vector<Vec> ch(2 * n);
    // right
    for (int i = 0; i < n; ++i) {
        while (k > 1 && leq(cross(ch[k-1] - ch[k-2], pts[i] - ch[k-1]), 0)) --k;
        ch[k++] = pts[i];
    }
    int t = k;
    // left
    for (int i = n - 2; i >= 0; --i) {
        while (k > t && leq(cross(ch[k-1] - ch[k-2], pts[i] - ch[k-1]), 0)) --k;
        ch[k++] = pts[i];
    }
    ch.resize(k - 1);
    return ch;
}


int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(15);

    int n;
    cin >> n;
    vector<Vec> pts(n);
    for (auto& x : pts) cin >> x;
    rep(i,0,n-1) {
        vector<Vec> first, second;
        rep(j,0,n) {
            if (j <= i) first.push_back(pts[j]);
            else second.push_back(pts[j]);
        }
        first = convex_hull(first);
        second = convex_hull(second);
        // if (intersect(first, second)) {
        //     cout << "Impossible" << endl;
        //     return 0;
        // }
        int m = first.size();
        bool ok = false;
        rep(j,0,m) {
            if (first[j] == pts[i]) {
                if (!separate(first, second, Line(first[(j-1+m)%m], first[j])) &&
                    !separate(first, second, Line(first[j], first[(j+1)%m]))) {

                    cout << "Impossible" << endl;
                    return 0;
                }
                ok = true;
            }
        }
        if (!ok) {
            cout << "Impossible" << endl;
            return 0;
        }
        m = second.size();
        ok = false;
        rep(j,0,m) {
            if (second[j] == pts[i+1]) {
                if (!separate(first, second, Line(second[(j-1+m)%m], second[j])) &&
                    !separate(first, second, Line(second[j], second[(j+1)%m]))) {

                    cout << "Impossible" << endl;
                    return 0;
                }
                ok = true;
            }
        }
        if (!ok) {
            cout << "Impossible" << endl;
            return 0;
        }
    }
    cout << "Possible" << endl;
}
accuracy: 0.014705882352941176
time_ms: 30
memory_kb: 3660
final_rank: 15

code_snippet: #include <bits/stdc++.h>

using namespace std;

#define eps 1e-9
#define INF 2000000000              // 2e9
#define LLINF 2000000000000000000ll // 2e18 (llmax:9e18)
#define all(x) (x).begin(), (x).end()
#define sq(x) ((x) * (x))

#define rep(i, x) for (int i = 0; i < (int)(x); i++)
#define drep(i, x) for (int i = ((int)(x)-1); i >= 0; i--)

#define popcount __builtin_popcount
#define next __next
#define prev __prev

#ifndef LOCAL
#define dmp(...) ;
#else
#define dmp(...)                                                               \
  cerr << "[ " << #__VA_ARGS__ << " ] : " << dump_str(__VA_ARGS__) << endl
#endif

// ---------------- Utility ------------------

template <class T>
bool chmin(T &a, const T &b) {
  if (a <= b) return false;
  a = b;
  return true;
}

template <class T>
bool chmax(T &a, const T &b) {
  if (a >= b) return false;
  a = b;
  return true;
}

template <class T>
using MaxHeap = priority_queue<T>;

template <class T>
using MinHeap = priority_queue<T, vector<T>, greater<T>>;

template <class T>
vector<T> vect(int len, T elem) {
  return vector<T>(len, elem);
}

// ----------------- Input -------------------

template <class T, class U>
istream &operator>>(istream &is, pair<T, U> &p) {
  return is >> p.first >> p.second;
}

template <class T>
istream &operator>>(istream &is, vector<T> &vec) {
  for (int i = 0; i < vec.size(); i++) is >> vec[i];
  return is;
}

// ----------------- Output ------------------

template <class T, class U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  return os << p.first << ',' << p.second;
}

template <class T>
ostream &operator<<(ostream &os, const vector<T> &v) {
  for (const T &e : v) os << e << " ";
  return os;
}

template <class T>
ostream &operator<<(ostream &os, const deque<T> &d) {
  for (const T &e : d) os << e << " ";
  return os;
}

template <class T>
ostream &operator<<(ostream &os, const set<T> &s) {
  os << "{ ";
  for (const T &e : s) os << e << " ";
  return os << "}";
}

template <class T, class U>
ostream &operator<<(ostream &os, const map<T, U> &m) {
  os << "{ ";
  for (const auto &[key, val] : m) os << "( " << key << " -> " << val << " ) ";
  return os << "}";
}

template <class TupleTy, size_t... I>
void dump_tuple(ostream &os, const TupleTy t, std::index_sequence<I...>) {
  (..., (os << (I == 0 ? "" : ",") << std::get<I>(t)));
}

template <class... Args>
ostream &operator<<(ostream &os, const tuple<Args...> &t) {
  os << "(";
  dump_tuple(os, t, std::make_index_sequence<sizeof...(Args)>());
  return os << ")";
}

void dump_str_rec(ostringstream &) {}

template <class Head, class... Tail>
void dump_str_rec(ostringstream &oss, Head &&head, Tail &&... tail) {
  oss << ", " << head;
  dump_str_rec(oss, forward<Tail>(tail)...);
}

template <class T, class... U>
string dump_str(T &&arg, U &&... args) {
  ostringstream oss;
  oss << arg;
  dump_str_rec(oss, forward<U>(args)...);
  return oss.str();
}

// --------------- Fast I/O ------------------

void fastio() {
  cin.tie(0);
  ios::sync_with_stdio(0);
  cout << fixed << setprecision(20);
}

// ------------------ ACL --------------------

// #include <atcoder/modint>
// constexpr int MOD = 998244353;
// constexpr int MOD = 1000000007;
// using mint = atcoder::static_modint<MOD>;
// ostream &operator<<(ostream &os, const mint &v) {
//   return os << v.val();
// }

// ------------ End of template --------------

// Geometry Library
// written by okuraofvegetable

#define pb push_back
#define DOUBLE_INF 1e50
#define Points vector<Point>

#define EQ(a, b) (abs((a) - (b)) < eps)
#define GT(a, b) ((a) > (b))
#define GE(a, b) (EQ(a, b) || GT(a, b))
#define LT(a, b) ((a) < (b))
#define LE(a, b) (EQ(a, b) || LT(a, b))

inline double add(double a, double b) {
  if (abs(a + b) < eps * (abs(a) + abs(b))) return 0;
  return a + b;
}

// --------------------- Point -----------------------

#define Vector Point
struct Point {
  double x, y;
  Point() {}
  Point(double x, double y) : x(x), y(y) {}
  Point operator+(Point p) { return Point(add(x, p.x), add(y, p.y)); }
  Point operator-(Point p) { return Point(add(x, -p.x), add(y, -p.y)); }
  Point operator*(double d) { return Point(x * d, y * d); }
  Point operator/(double d) { return Point(x / d, y / d); }
  double dot(Point p) { return add(x * p.x, y * p.y); }
  double det(Point p) { return add(x * p.y, -y * p.x); }
  double norm() { return sqrt(x * x + y * y); }
  double norm2() { return x * x + y * y; }
  double dist(Point p) { return ((*this) - p).norm(); }
  double dist2(Point p) { return sq(x - p.x) + sq(y - p.y); }
  double arg() { return atan2(y, x); }             // [-PI,PI]
  double arg(Vector v) { return v.arg() - arg(); } // henkaku
  double angle(Vector v) {                         // [0,PI]
    return acos(cos(arg(v)));
  }
  Vector normalize() { return (*this) * (1.0 / norm()); }
  Point vert() { return Point(y, -x); }

  // Signed area of triange (0,0) (x,y) (p.x,p.y)
  double area(Point p) { return (x * p.y - p.x * y) / 2.0; }
  friend istream &operator>>(istream &is, Point &p) { return is >> p.x >> p.y; }
  friend ostream &operator<<(ostream &os, const Point &p) {
    return os << '(' << p.x << ',' << p.y << ')';
  }
};

Point divide_rate(Point a, Point b, double A, double B) {
  assert(!EQ(A + B, 0.0));
  return (a * B + b * A) * (1.0 / (A + B));
}

Vector polar(double len, double arg) {
  return Vector(cos(arg) * len, sin(arg) * len);
}

// Direction a -> b -> c
// verified AOJ CGL_1_C
enum { COUNTER_CLOCKWISE, CLOCKWISE, ONLINE_BACK, ONLINE_FRONT, ON_SEGMENT };
int ccw(Point a, Point b, Point c) {
  Vector p = b - a;
  Vector q = c - a;
  if (p.det(q) > 0.0) return COUNTER_CLOCKWISE; // counter clockwise
  if (p.det(q) < 0.0) return CLOCKWISE;         // clockwise
  if (p.dot(q) < 0.0) return ONLINE_BACK;       // c--a--b online_back
  if (p.norm() < q.norm()) return ONLINE_FRONT; // a--b--c online_front
  return ON_SEGMENT;                            // a--c--b on_segment
}

// --------------------- Line ------------------------
struct Line {
  Point a, b;
  Line() {}
  Line(Point a, Point b) : a(a), b(b) {}
  bool on(Point q) { return EQ((a - q).det(b - q), 0.0); }
  // following 2 functions verified AOJ CGL_2_A
  bool is_parallel(Line l) { return EQ((a - b).det(l.a - l.b), 0.0); }
  bool is_orthogonal(Line l) { return EQ((a - b).dot(l.a - l.b), 0.0); }
  Point intersection(Line l) {
    assert(!is_parallel(l));
    return a + (b - a) * ((l.b - l.a).det(l.a - a) / (l.b - l.a).det(b - a));
  }
  // Projection of p to this line
  // verified AOJ CGL_1_A
  Point projection(Point p) {
    return (b - a) * ((b - a).dot(p - a) / (b - a).norm2()) + a;
  }
  double distance(Point p) {
    Point q = projection(p);
    return p.dist(q);
  }
  // Reflection point of p onto this line
  // verified AOJ CGL_1_B
  Point refl(Point p) {
    Point proj = projection(p);
    return p + ((proj - p) * 2.0);
  }
  bool left(Point p) {
    if ((p - a).det(b - a) > 0.0)
      return true;
    else
      return false;
  }
  friend istream &operator>>(istream &is, Line &l) { return is >> l.a >> l.b; }
  friend ostream &operator<<(ostream &os, const Line &l) {
    return os << l.a << " -> " << l.b;
  }
};

// ------------------- Segment  ----------------------
struct Segment {
  Point a, b;
  Segment() {}
  Segment(Point a, Point b) : a(a), b(b) {}
  Line line() { return Line(a, b); }
  bool on(Point q) {
    return (EQ((a - q).det(b - q), 0.0) && LE((a - q).dot(b - q), 0.0));
  }
  // verified AOJ CGL_2_B
  bool is_intersect(Segment s) {
    if (line().is_parallel(s.line())) {
      if (on(s.a) || on(s.b)) return true;
      if (s.on(a) || s.on(b)) return true;
      return false;
    }
    Point p = line().intersection(s.line());
    if (on(p) && s.on(p))
      return true;
    else
      return false;
  }
  bool is_intersect(Line l) {
    if (line().is_parallel(l)) {
      if (l.on(a) || l.on(b))
        return true;
      else
        return false;
    }
    Point p = line().intersection(l);
    if (on(p))
      return true;
    else
      return false;
  }
  // following 2 distance functions are verified at AOJ CGL_2_D
  double distance(Point p) {
    double res = DOUBLE_INF;
    Point q = line().projection(p);
    if (on(q)) res = min(res, p.dist(q));
    res = min(res, min(p.dist(a), p.dist(b)));
    return res;
  }
  double distance(Line l) {
    if (is_intersect(l)) return 0.0;
    double res = DOUBLE_INF;
    res = min(res, l.distance(a));
    res = min(res, l.distance(b));
    return res;
  }
  double distance(Segment s) {
    if (is_intersect(s)) return 0.0;
    double res = DOUBLE_INF;
    res = min(res, s.distance(a));
    res = min(res, s.distance(b));
    res = min(res, this->distance(s.a));
    res = min(res, this->distance(s.b));
    return res;
  }
  friend istream &operator>>(istream &is, Segment &s) {
    return is >> s.a >> s.b;
  }
  friend ostream &operator<<(ostream &os, const Segment &s) {
    return os << s.a << " -> " << s.b;
  }
};

// ------------------- Polygon -----------------------
// Note: A polygon generally don't need to be convex.

typedef vector<Point> Polygon;
// verified AOJ CGL_3_A
double area(Polygon &pol) {
  Points vec;
  double res = 0.0;
  int M = pol.size();
  for (int i = 0; i < M; i++) {
    res += (pol[i] - pol[0]).area(pol[(i + 1) % M] - pol[0]);
  }
  return res;
}

bool is_convex(Polygon &pol) {
  int n = pol.size();
  for (int i = 0; i < n - 1; i++) {
    if (ccw(pol[i], pol[i + 1], pol[(i + 2) % n]) == CLOCKWISE) {
      return false;
    }
  }
  return true;
}

// vecrified AOJ CGL_3_C
enum { OUT, ON, IN };
int contained(Polygon &pol, Point p) {
  int n = pol.size();
  Point outer(1e9, p.y);
  Segment s = Segment(outer, p);
  int cnt = 0;
  for (int i = 0; i < n; i++) {
    Segment e = Segment(pol[i], pol[(i + 1) % n]);
    if (e.on(p)) return ON;
    Vector a = pol[i] - p;
    Vector b = pol[(i + 1) % n] - p;
    if (a.y > b.y) swap(a, b);
    if (a.y <= 0.0 && b.y > 0.0) {
      if (a.det(b) < 0.0) cnt++;
    }
  }
  if ((cnt & 1) == 1)
    return IN;
  else
    return OUT;
}

// A compare function for convex_hull
// sort points by (x-y) lexicographical order.
// you can change (y-x) order with no change in convex_hull
bool comp(const Point &p, const Point &q) {
  if (!EQ(p.x, q.x))
    return p.x < q.x;
  else
    return p.y < q.y;
}

// Convex hull
// if you want not to contain points on boundary,
// change while(....<=0.0)
// verified AOJ CGL_4_A
Polygon convex_hull(vector<Point> ps) {
  sort(all(ps), comp);
  int k = 0;
  int n = ps.size();
  Polygon qs(2 * n);
  for (int i = 0; i < n; i++) {
    while (k > 1 && (qs[k - 1] - qs[k - 2]).det(ps[i] - qs[k - 1]) < 0.0) k--;
    qs[k++] = ps[i];
  }
  for (int i = n - 2, t = k; i >= 0; i--) {
    while (k > t && (qs[k - 1] - qs[k - 2]).det(ps[i] - qs[k - 1]) < 0.0) k--;
    qs[k++] = ps[i];
  }
  qs.resize(k - 1);
  return qs;
}

// Caliper method
// verified AOJ CGL_4_B
double convex_diameter(Polygon &cv) {
  int i = 0, j = 0;
  int n = cv.size();
  for (int k = 0; k < n; k++) {
    if (!comp(cv[i], cv[k])) i = k;
    if (comp(cv[j], cv[k])) j = k;
  }
  int si = i, sj = j;
  double res = 0.0;
  while (i != sj || j != si) {
    res = max(res, cv[i].dist(cv[j]));
    if ((cv[(i + 1) % n] - cv[i]).det(cv[(j + 1) % n] - cv[j]) < 0.0)
      i = (i + 1) % n;
    else
      j = (j + 1) % n;
  }
  return res;
}

// Cut conovex polygon by a line and return left polygon
// verified AOJ CGL_4_C
Polygon convex_cut(Polygon &cv, Line l) {
  int n = cv.size();
  Polygon left;
  for (int i = 0; i < n; i++) {
    Segment e = Segment(cv[i], cv[(i + 1) % n]);
    if (ccw(l.a, l.b, cv[i]) != CLOCKWISE) left.pb(cv[i]);
    if (e.is_intersect(l)) {
      if (!e.line().is_parallel(l)) { left.pb(e.line().intersection(l)); }
    }
  }
  return left;
}

// Distance between closest pair
// verified CGL_5_A
bool comp_y(const Point &p, const Point &q) {
  return p.y < q.y;
}
double closest_pair(vector<Point>::iterator a, int n) {
  if (n <= 1) return DOUBLE_INF;
  int m = n / 2;
  double x = (a + m)->x;
  double d = min(closest_pair(a, m), closest_pair(a + m, n - m));
  inplace_merge(a, a + m, a + n, comp_y);
  vector<Point> b;
  for (int i = 0; i < n; i++) {
    double ax = (a + i)->x;
    double ay = (a + i)->y;
    if (abs(ax - x) >= d) continue;
    for (int j = 0; j < b.size(); j++) {
      double dx = ax - b[b.size() - 1 - j].x;
      double dy = ay - b[b.size() - 1 - j].y;
      if (dy >= d) break;
      d = min(d, sqrt(dx * dx + dy * dy));
    }
    b.pb(*(a + i));
  }
  return d;
}
double closest_pair(vector<Point> a) {
  sort(all(a), comp);
  return closest_pair(a.begin(), (int)a.size());
}

#define endl "\n"
using ll = long long;
using pii = pair<int, int>;

const bool multitest = false;

bool solve() {
  int n;
  cin >> n;
  vector<Point> ps(n);
  cin >> ps;
  {
    vector<Point> cur;
    for (int i = 0; i < n; i++) {
      cur.push_back(ps[i]);
      if (cur.size() >= 3 && i + 1 < n) {
        Polygon hull = convex_hull(cur);
        if (contained(hull, ps[i + 1]) == IN) {
          cout << "Impossible" << endl;
          return true;
        }
      }
    }
  }
  reverse(all(ps));
  {
    vector<Point> cur;
    for (int i = 0; i < n; i++) {
      cur.push_back(ps[i]);
      if (cur.size() >= 3 && i + 1 < n) {
        Polygon hull = convex_hull(cur);
        if (contained(hull, ps[i + 1]) == IN) {
          cout << "Impossible" << endl;
          return true;
        }
      }
    }
  }
  cout << "Possible" << endl;
  return true;
}

int main() {
  fastio();
  if (!multitest) {
    solve();
  } else {
    cerr << "[Warning] Multi testcase mode on" << endl;
    int t;
    cin >> t;
    while (t--) solve();
  }
  return 0;
}
accuracy: 1
time_ms: 60
memory_kb: 3664
final_rank: 6

code_snippet: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=305,INF=1<<29;
#define double ll

//幾何ライブラリ
// define double ll をするときは Point の < と == も書き換えよう！

const double eps=1e-8;
const double pi=acos((double)-1.0L);
#define equals(a,b) (fabs((a)-(b))<eps)

double torad(double deg) {return (double)(deg)*pi/180.0;}
double todeg(double ang) {return ang*180.0/pi;}

class Point{
public:
    double x,y;
    
    Point(double x=0,double y=0):x(x),y(y){}
    
    Point operator + (Point p){return Point(x+p.x,y+p.y);}
    Point operator - (Point p){return Point(x-p.x,y-p.y);}
    Point operator * (double a){return Point(a*x,a*y);}
    Point operator / (double a){return Point(x/a,y/a);}
    
    double abs(){return sqrt(norm());}
    double norm(){return x*x+y*y;}
    
    bool operator < (const Point &p)const{
        return x<p.x||(x==p.x&&y<p.y);
    }
    
    bool operator == (const Point &p)const{
        return x==p.x&&y==p.y;
    }
};

typedef Point Vector;

double norm(Vector a){
    return a.x*a.x+a.y*a.y;
}

double abs(Vector a){
    return sqrt(norm(a));
}

double dot(Vector a,Vector b){
    return a.x*b.x+a.y*b.y;
}

double cross(Vector a,Vector b){
    return a.x*b.y-a.y*b.x;
}

struct Segment{
    Point p1,p2;
};

bool isOrthogonal(Vector a,Vector b){
    return equals(dot(a,b),0.0);
}

bool isOrthogonal(Point a1,Point a2,Point b1,Point b2){
    return isOrthogonal(a1-a2,b1-b2);
}

bool isOrthogonal(Segment s1,Segment s2){
    return equals(dot(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

bool isParallel(Vector a,Vector b){
    return equals(cross(a,b),0.0);
}

bool isParallel(Point a1,Point a2,Point b1,Point b2){
    return isParallel(a1-a2,b1-b2);
}

bool isParallel(Segment s1,Segment s2){
    return equals(cross(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

Point project(Segment s,Point p){
    Vector base=s.p2-s.p1;
    double r=dot(p-s.p1,base)/norm(base);
    return s.p1+base*r;
}

Point reflect(Segment s,Point p){
    return p+(project(s,p)-p)*2.0;
}

//p0,p1,p2の順に見たときどうなるか？

static const int counter_clockwise=1;
static const int clockwise=-1;
static const int online_back=2;
static const int online_front=-2;
static const int on_segment=0;

int ccw(Point p0,Point p1,Point p2){
    Vector a=p1-p0;
    Vector b=p2-p0;
    
    if(cross(a,b)>eps) return counter_clockwise;
    if(cross(a,b)<-eps) return clockwise;
    if(dot(a,b)<-eps) return online_back;
    if(a.norm()<b.norm()) return online_front;
    
    return on_segment;
}

bool intersect(Point p1,Point p2,Point p3,Point p4){
    return(ccw(p1,p2,p3)*ccw(p1,p2,p4)<=0&&ccw(p3,p4,p1)*ccw(p3,p4,p2)<=0);
}

bool intersect(Segment s1,Segment s2){
    return intersect(s1.p1,s1.p2,s2.p1,s2.p2);
}

bool overlap(Segment s1,Segment s2){
    int a=ccw(s1.p1,s1.p2,s2.p1),b=ccw(s1.p1,s1.p2,s2.p2);
    if(a&1||b&1) return 0;
    if(a==2){
        if(b==-2||(b==0&&!(s2.p2==s1.p1))) return 1;
        else return 0;
    }
    if(a==-2){
        if(b==2||(b==0&&!(s2.p2==s1.p2))) return 1;
        else return 0;
    }
    if(a==0){
        if(s1.p1==s2.p1){
            if(b!=2) return 1;
            else return 0;
        }
        else if(s1.p2==s2.p1){
            if(b!=-2) return 1;
            else return 0;
        }
        else return 1;
    }
    return 0;
}
//s1とs2の共通の線分(長さ0より大きい)があるかどうか

typedef Segment Line;

double getDistance(Point a,Point b){
    return abs(a-b);
}

double getDistanceLP(Line l,Point p){
    return abs(cross(l.p2-l.p1,p-l.p1)/abs(l.p2-l.p1));
}

double getDistanceSP(Segment s,Point p){
    if(dot(s.p2-s.p1,p-s.p1)<0.0) return abs(p-s.p1);
    if(dot(s.p1-s.p2,p-s.p2)<0.0) return abs(p-s.p2);
    return getDistanceLP(s,p);
}

double getDistance(Segment s1,Segment s2){
    if(intersect(s1,s2)) return 0.0;
    return min({getDistanceSP(s1,s2.p1),getDistanceSP(s1,s2.p2),getDistanceSP(s2,s1.p1),getDistanceSP(s2,s1.p2)});
}

Point getCrossPointS(Segment s1,Segment s2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    Vector base=s2.p2-s2.p1;
    double d1=abs(cross(base,s1.p1-s2.p1));
    double d2=abs(cross(base,s1.p2-s2.p1));
    double t=d1/(d1+d2);
    return s1.p1+(s1.p2-s1.p1)*t;
}//同じ時壊れます

Point getCrossPointL(Line l1,Line l2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    
    Vector v1=l1.p2-l1.p1,v2=l2.p2-l2.p1;
    
    return l1.p1+v1*cross(v2,l2.p1-l1.p1)/cross(v2,v1);
}

Segment ParallelSegment(Segment s,double d){
    Vector v={-(s.p2-s.p1).y,(s.p2-s.p1).x};
    v=v/abs(v);
    
    s.p1=s.p1+v*d;
    s.p2=s.p2+v*d;
    
    return s;
}

Point naisin(Point p1,Point p2,Point p3){
    if(p1==p2&&p2==p3&&p3==p1) return p1;
    
    return (p1*abs(p2-p3)+p2*abs(p1-p3)+p3*abs(p1-p2))/(abs(p2-p3)+abs(p1-p3)+abs(p1-p2));
}

Point naisin(Line l1,Line l2,Line l3){
    //平行でない前提
    
    Point p1=getCrossPointL(l1,l2),p2=getCrossPointL(l1,l3),p3=getCrossPointL(l2,l3);
    return naisin(p1,p2,p3);
}

//ネットの適当を書いたのであってるか全く知りません→あってそう

class Circle{
public:
    Point c;
    double r;
    Circle(Point c=Point(),double r=0.0):c(c),r(r){}
};

Point CircleCenter(Point a,Point b,Point c){
    Point u=a-b,v=a-c;
    double m1=(norm(a)-norm(b))/2.0,m2=(norm(a)-norm(c))/2.0;
    
    Point res;
    if(cross(u,v)==0.0){
        res.x=1e9;
        res.y=1e9;
        
        return res;
    }
    res.x=(m1*v.y-m2*u.y)/cross(u,v);
    res.y=(m1*v.x-m2*u.x)/cross(v,u);
    
    return res;
}
//3点を通る円の中心を返す

//交わる 0
// c1がc2のinside 1
// c1がc2のoutside 2
// 交わらない 3

int not_intersect(Circle c1,Circle c2){
    double d=getDistance(c1.c,c2.c);
    double r1=c1.r,r2=c2.r;
    if(r1<r2){
        if(d<(r2-r1)) return 1;
    }
    if(r1>r2){
        if(d<(r1-r2)) return 2;
    }
    if(d<=r1+r2) return 0;
    else return 3;
}

pair<Point,Point> segCrossPpoints(Circle c,Line l){
    //assert(intersect(c,l));
    Vector pr=project(l,c.c);
    Vector e=(l.p2-l.p1)/abs(l.p2-l.p1);
    double base=sqrt(c.r*c.r-norm(pr-c.c));
    return make_pair(pr+e*base,pr-e*base);
}

double arg(Vector p){return atan2(p.y,p.x);}
Vector polar(double a,double r){return Point(cos(r)*a,sin(r)*a);}

//inside(outside)

pair<Point,Point> getCrossPoints(Circle c1,Circle c2){
    //assert(intersect(c1,c2));
    double d=abs(c1.c-c2.c);
    double a=acos((c1.r*c1.r+d*d-c2.r*c2.r)/(2*c1.r*d));
    double t=arg(c2.c-c1.c);
    return make_pair(c1.c+polar(c1.r,t+a),c1.c+polar(c1.r,t-a));
}

vector<Line> Commontangent(Circle c1,Circle c2){
    vector<Line> res;
    Point p=c2.c-c1.c;
    
    if(abs(p)>=(c1.r+c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r+c2.r)+p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r+c2.r)-p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r+c2.r)-p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r+c2.r)+p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    if(abs(p)>=abs(c1.r-c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r-c2.r)+p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r-c2.r)-p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r-c2.r)-p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r-c2.r)+p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    return res;
}

typedef vector<Point> Polygon;

/*
 IN 2
 ON 1
 OUT 0
 */

int contains(Polygon g,Point p){
    int n=int(g.size());
    bool x=false;
    for(int i=0;i<n;i++){
        Point a=g[i]-p,b=g[(i+1)%n]-p;
        if(a.y>b.y) swap(a,b);
        if(a.y<=0&&0<b.y&&cross(a,b)<0) x=!x;
        if(abs(cross(a,b))<=0&&dot(a,b)<=0) return 1;
    }
    return (x?2:0);
}

Polygon andrewScan(Polygon s,bool ok){
    Polygon u,l;
    sort(all(s));
    s.erase(unique(all(s)),s.end());
    
    if(int(s.size())<3) return s;
    int n=int(s.size());
    
    u.push_back(s[0]);
    u.push_back(s[1]);
    
    l.push_back(s[n-1]);
    l.push_back(s[n-2]);
    
    if(ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])==counter_clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])==counter_clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    if(!ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])!=clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])!=clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    reverse(all(l));
    
    for(int i=int(u.size())-2;i>=1;i--) l.push_back(u[i]);
    
    return l;
}//ok==1なら辺の上も含める

Polygon convex_cut(const Polygon& P, const Line& l) {
    Polygon Q;
    for(int i=0;i<si(P);i++){
        Point A=P[i],B=P[(i+1)%si(P)];
        if(ccw(l.p1,l.p2,A)!=-1)Q.push_back(A);
        if(ccw(l.p1,l.p2,A)*ccw(l.p1,l.p2,B)<0) Q.push_back(getCrossPointL(Line{A,B},l));
    }
    return Q;
}

double area(Point a,Point b,Point c){
    b=b-a;
    c=c-a;
    return abs(b.x*c.y-b.y*c.x)/2.0;
}

ll gcd(ll a,ll b){
    if(b==0) return a;
    return gcd(b,a%b);
}

pair<Point,Vector> perpendicular_bisector(Point a,Point b){
    Point c=(a+b)/2;
    Vector v=b-c;
    swap(v.x,v.y);
    v.x*=-1;
    
    Point p=c;
    if(v.x==0){
        v.y=1;
        p.y=0;
    }
    else if(v.y==0){
        v.x=1;
        p.x=0;
    }
    else{
        if(v.x<0){
            v.x*=-1;
            v.y*=-1;
        }
        ll g=gcd(abs(ll(v.x)),abs(ll(v.y)));
        v.x/=g;
        v.y/=g;
        if(p.x>=0){
            ll d=p.x/v.x;
            p=p-v*d;
        }else{
            ll d=abs(p.x)/v.x;
            p=p+v*d;
            
            if(p.x<0){
                p=p+v;
            }
        }
    }
    
    return mp(p,v);
}
//2倍するなりして整数にしておくこと

int main() {
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int N;cin>>N;
    vector<Point> P(N);
    for(int i=0;i<N;i++) cin>>P[i].x>>P[i].y;
    bool f=true;
    for(int i=0;i<N;i++){
        vector<Point> A,B;
        for(int j=0;j<i;j++) A.push_back(P[j]);
        for(int j=i+1;j<N;j++) B.push_back(P[j]);
        A=andrewScan(A,0);
        B=andrewScan(B,0);
        if(contains(A,P[i])) f=false;
        if(contains(B,P[i])) f=false;
    }
    if(f) cout<<"Possible\n";
    else cout<<"Impossible\n";
}
accuracy: 1
time_ms: 40
memory_kb: 3608
final_rank: 2

code_snippet: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=305,INF=1<<29;
#define double ll

//幾何ライブラリ
// define double ll をするときは Point の < と == も書き換えよう！

const double eps=1e-8;
const double pi=acos((double)-1.0L);
#define equals(a,b) (fabs((a)-(b))<eps)

double torad(double deg) {return (double)(deg)*pi/180.0;}
double todeg(double ang) {return ang*180.0/pi;}

class Point{
public:
    double x,y;
    
    Point(double x=0,double y=0):x(x),y(y){}
    
    Point operator + (Point p){return Point(x+p.x,y+p.y);}
    Point operator - (Point p){return Point(x-p.x,y-p.y);}
    Point operator * (double a){return Point(a*x,a*y);}
    Point operator / (double a){return Point(x/a,y/a);}
    
    double abs(){return sqrt(norm());}
    double norm(){return x*x+y*y;}
    
    bool operator < (const Point &p)const{
        return x<p.x||(x==p.x&&y<p.y);
    }
    
    bool operator == (const Point &p)const{
        return x==p.x&&y==p.y;
    }
};

typedef Point Vector;

double norm(Vector a){
    return a.x*a.x+a.y*a.y;
}

double abs(Vector a){
    return sqrt(norm(a));
}

double dot(Vector a,Vector b){
    return a.x*b.x+a.y*b.y;
}

double cross(Vector a,Vector b){
    return a.x*b.y-a.y*b.x;
}

struct Segment{
    Point p1,p2;
};

bool isOrthogonal(Vector a,Vector b){
    return equals(dot(a,b),0.0);
}

bool isOrthogonal(Point a1,Point a2,Point b1,Point b2){
    return isOrthogonal(a1-a2,b1-b2);
}

bool isOrthogonal(Segment s1,Segment s2){
    return equals(dot(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

bool isParallel(Vector a,Vector b){
    return equals(cross(a,b),0.0);
}

bool isParallel(Point a1,Point a2,Point b1,Point b2){
    return isParallel(a1-a2,b1-b2);
}

bool isParallel(Segment s1,Segment s2){
    return equals(cross(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

Point project(Segment s,Point p){
    Vector base=s.p2-s.p1;
    double r=dot(p-s.p1,base)/norm(base);
    return s.p1+base*r;
}

Point reflect(Segment s,Point p){
    return p+(project(s,p)-p)*2.0;
}

//p0,p1,p2の順に見たときどうなるか？

static const int counter_clockwise=1;
static const int clockwise=-1;
static const int online_back=2;
static const int online_front=-2;
static const int on_segment=0;

int ccw(Point p0,Point p1,Point p2){
    Vector a=p1-p0;
    Vector b=p2-p0;
    
    if(cross(a,b)>eps) return counter_clockwise;
    if(cross(a,b)<-eps) return clockwise;
    if(dot(a,b)<-eps) return online_back;
    if(a.norm()<b.norm()) return online_front;
    
    return on_segment;
}

bool intersect(Point p1,Point p2,Point p3,Point p4){
    return(ccw(p1,p2,p3)*ccw(p1,p2,p4)<=0&&ccw(p3,p4,p1)*ccw(p3,p4,p2)<=0);
}

bool intersect(Segment s1,Segment s2){
    return intersect(s1.p1,s1.p2,s2.p1,s2.p2);
}

bool overlap(Segment s1,Segment s2){
    int a=ccw(s1.p1,s1.p2,s2.p1),b=ccw(s1.p1,s1.p2,s2.p2);
    if(a&1||b&1) return 0;
    if(a==2){
        if(b==-2||(b==0&&!(s2.p2==s1.p1))) return 1;
        else return 0;
    }
    if(a==-2){
        if(b==2||(b==0&&!(s2.p2==s1.p2))) return 1;
        else return 0;
    }
    if(a==0){
        if(s1.p1==s2.p1){
            if(b!=2) return 1;
            else return 0;
        }
        else if(s1.p2==s2.p1){
            if(b!=-2) return 1;
            else return 0;
        }
        else return 1;
    }
    return 0;
}
//s1とs2の共通の線分(長さ0より大きい)があるかどうか

typedef Segment Line;

double getDistance(Point a,Point b){
    return abs(a-b);
}

double getDistanceLP(Line l,Point p){
    return abs(cross(l.p2-l.p1,p-l.p1)/abs(l.p2-l.p1));
}

double getDistanceSP(Segment s,Point p){
    if(dot(s.p2-s.p1,p-s.p1)<0.0) return abs(p-s.p1);
    if(dot(s.p1-s.p2,p-s.p2)<0.0) return abs(p-s.p2);
    return getDistanceLP(s,p);
}

double getDistance(Segment s1,Segment s2){
    if(intersect(s1,s2)) return 0.0;
    return min({getDistanceSP(s1,s2.p1),getDistanceSP(s1,s2.p2),getDistanceSP(s2,s1.p1),getDistanceSP(s2,s1.p2)});
}

Point getCrossPointS(Segment s1,Segment s2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    Vector base=s2.p2-s2.p1;
    double d1=abs(cross(base,s1.p1-s2.p1));
    double d2=abs(cross(base,s1.p2-s2.p1));
    double t=d1/(d1+d2);
    return s1.p1+(s1.p2-s1.p1)*t;
}//同じ時壊れます

Point getCrossPointL(Line l1,Line l2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    
    Vector v1=l1.p2-l1.p1,v2=l2.p2-l2.p1;
    
    return l1.p1+v1*cross(v2,l2.p1-l1.p1)/cross(v2,v1);
}

Segment ParallelSegment(Segment s,double d){
    Vector v={-(s.p2-s.p1).y,(s.p2-s.p1).x};
    v=v/abs(v);
    
    s.p1=s.p1+v*d;
    s.p2=s.p2+v*d;
    
    return s;
}

Point naisin(Point p1,Point p2,Point p3){
    if(p1==p2&&p2==p3&&p3==p1) return p1;
    
    return (p1*abs(p2-p3)+p2*abs(p1-p3)+p3*abs(p1-p2))/(abs(p2-p3)+abs(p1-p3)+abs(p1-p2));
}

Point naisin(Line l1,Line l2,Line l3){
    //平行でない前提
    
    Point p1=getCrossPointL(l1,l2),p2=getCrossPointL(l1,l3),p3=getCrossPointL(l2,l3);
    return naisin(p1,p2,p3);
}

//ネットの適当を書いたのであってるか全く知りません→あってそう

class Circle{
public:
    Point c;
    double r;
    Circle(Point c=Point(),double r=0.0):c(c),r(r){}
};

Point CircleCenter(Point a,Point b,Point c){
    Point u=a-b,v=a-c;
    double m1=(norm(a)-norm(b))/2.0,m2=(norm(a)-norm(c))/2.0;
    
    Point res;
    if(cross(u,v)==0.0){
        res.x=1e9;
        res.y=1e9;
        
        return res;
    }
    res.x=(m1*v.y-m2*u.y)/cross(u,v);
    res.y=(m1*v.x-m2*u.x)/cross(v,u);
    
    return res;
}
//3点を通る円の中心を返す

//交わる 0
// c1がc2のinside 1
// c1がc2のoutside 2
// 交わらない 3

int not_intersect(Circle c1,Circle c2){
    double d=getDistance(c1.c,c2.c);
    double r1=c1.r,r2=c2.r;
    if(r1<r2){
        if(d<(r2-r1)) return 1;
    }
    if(r1>r2){
        if(d<(r1-r2)) return 2;
    }
    if(d<=r1+r2) return 0;
    else return 3;
}

pair<Point,Point> segCrossPpoints(Circle c,Line l){
    //assert(intersect(c,l));
    Vector pr=project(l,c.c);
    Vector e=(l.p2-l.p1)/abs(l.p2-l.p1);
    double base=sqrt(c.r*c.r-norm(pr-c.c));
    return make_pair(pr+e*base,pr-e*base);
}

double arg(Vector p){return atan2(p.y,p.x);}
Vector polar(double a,double r){return Point(cos(r)*a,sin(r)*a);}

//inside(outside)

pair<Point,Point> getCrossPoints(Circle c1,Circle c2){
    //assert(intersect(c1,c2));
    double d=abs(c1.c-c2.c);
    double a=acos((c1.r*c1.r+d*d-c2.r*c2.r)/(2*c1.r*d));
    double t=arg(c2.c-c1.c);
    return make_pair(c1.c+polar(c1.r,t+a),c1.c+polar(c1.r,t-a));
}

vector<Line> Commontangent(Circle c1,Circle c2){
    vector<Line> res;
    Point p=c2.c-c1.c;
    
    if(abs(p)>=(c1.r+c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r+c2.r)+p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r+c2.r)-p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r+c2.r)-p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r+c2.r)+p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    if(abs(p)>=abs(c1.r-c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r-c2.r)+p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r-c2.r)-p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r-c2.r)-p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r-c2.r)+p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    return res;
}

typedef vector<Point> Polygon;

/*
 IN 2
 ON 1
 OUT 0
 */

int contains(Polygon g,Point p){
    int n=int(g.size());
    bool x=false;
    for(int i=0;i<n;i++){
        Point a=g[i]-p,b=g[(i+1)%n]-p;
        if(a.y>b.y) swap(a,b);
        if(a.y<=0&&0<b.y&&cross(a,b)<0) x=!x;
        if(abs(cross(a,b))<=0&&dot(a,b)<=0) return 1;
    }
    return (x?2:0);
}

Polygon andrewScan(Polygon s,bool ok){
    Polygon u,l;
    sort(all(s));
    s.erase(unique(all(s)),s.end());
    
    if(int(s.size())<3) return s;
    int n=int(s.size());
    
    u.push_back(s[0]);
    u.push_back(s[1]);
    
    l.push_back(s[n-1]);
    l.push_back(s[n-2]);
    
    if(ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])==counter_clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])==counter_clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    if(!ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])!=clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])!=clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    reverse(all(l));
    
    for(int i=int(u.size())-2;i>=1;i--) l.push_back(u[i]);
    
    return l;
}//ok==1なら辺の上も含める

Polygon convex_cut(const Polygon& P, const Line& l) {
    Polygon Q;
    for(int i=0;i<si(P);i++){
        Point A=P[i],B=P[(i+1)%si(P)];
        if(ccw(l.p1,l.p2,A)!=-1)Q.push_back(A);
        if(ccw(l.p1,l.p2,A)*ccw(l.p1,l.p2,B)<0) Q.push_back(getCrossPointL(Line{A,B},l));
    }
    return Q;
}

double area(Point a,Point b,Point c){
    b=b-a;
    c=c-a;
    return abs(b.x*c.y-b.y*c.x)/2.0;
}

ll gcd(ll a,ll b){
    if(b==0) return a;
    return gcd(b,a%b);
}

pair<Point,Vector> perpendicular_bisector(Point a,Point b){
    Point c=(a+b)/2;
    Vector v=b-c;
    swap(v.x,v.y);
    v.x*=-1;
    
    Point p=c;
    if(v.x==0){
        v.y=1;
        p.y=0;
    }
    else if(v.y==0){
        v.x=1;
        p.x=0;
    }
    else{
        if(v.x<0){
            v.x*=-1;
            v.y*=-1;
        }
        ll g=gcd(abs(ll(v.x)),abs(ll(v.y)));
        v.x/=g;
        v.y/=g;
        if(p.x>=0){
            ll d=p.x/v.x;
            p=p-v*d;
        }else{
            ll d=abs(p.x)/v.x;
            p=p+v*d;
            
            if(p.x<0){
                p=p+v;
            }
        }
    }
    
    return mp(p,v);
}
//2倍するなりして整数にしておくこと

int main() {
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int N;cin>>N;
    vector<Point> P(N);
    for(int i=0;i<N;i++) cin>>P[i].x>>P[i].y;
    bool f=false;
    for(int q=0;q<2;q++){
        bool ok=true;
        for(int i=2;i+1<N;i++){
            Polygon Q;
            for(int j=0;j<=i;j++) Q.push_back(P[j]);
            Q=andrewScan(Q,0);
            Point PP=P[i];
            bool aru=false;
            for(auto p:Q) if(p==PP) aru=true;
            if(!aru) ok=false;
            //if(contains(Q,PP)==2) ok=false;
        }
        for(int i=0;i+1<N-2;i++){
            Polygon Q;
            for(int j=i+1;j<N;j++) Q.push_back(P[j]);
            Q=andrewScan(Q,0);
            Point PP=P[i+1];
            bool aru=false;
            for(auto p:Q) if(p==PP) aru=true;
            if(!aru) ok=false;
            //if(contains(Q,PP)==2) ok=false;
        }
        if(ok) f=true;
        
        reverse(all(P));
    }
    if(f) cout<<"Possible\n";
    else cout<<"Impossible\n";
}
accuracy: 0.007352941176470588
time_ms: 40
memory_kb: 3424
final_rank: 16

code_snippet: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=305,INF=1<<29;
#define double ll

//幾何ライブラリ
// define double ll をするときは Point の < と == も書き換えよう！

const double eps=1e-8;
const double pi=acos((double)-1.0L);
#define equals(a,b) (fabs((a)-(b))<eps)

double torad(double deg) {return (double)(deg)*pi/180.0;}
double todeg(double ang) {return ang*180.0/pi;}

class Point{
public:
    double x,y;
    
    Point(double x=0,double y=0):x(x),y(y){}
    
    Point operator + (Point p){return Point(x+p.x,y+p.y);}
    Point operator - (Point p){return Point(x-p.x,y-p.y);}
    Point operator * (double a){return Point(a*x,a*y);}
    Point operator / (double a){return Point(x/a,y/a);}
    
    double abs(){return sqrt(norm());}
    double norm(){return x*x+y*y;}
    
    bool operator < (const Point &p)const{
        return x<p.x||(x==p.x&&y<p.y);
    }
    
    bool operator == (const Point &p)const{
        return x==p.x&&y==p.y;
    }
};

typedef Point Vector;

double norm(Vector a){
    return a.x*a.x+a.y*a.y;
}

double abs(Vector a){
    return sqrt(norm(a));
}

double dot(Vector a,Vector b){
    return a.x*b.x+a.y*b.y;
}

double cross(Vector a,Vector b){
    return a.x*b.y-a.y*b.x;
}

struct Segment{
    Point p1,p2;
};

bool isOrthogonal(Vector a,Vector b){
    return equals(dot(a,b),0.0);
}

bool isOrthogonal(Point a1,Point a2,Point b1,Point b2){
    return isOrthogonal(a1-a2,b1-b2);
}

bool isOrthogonal(Segment s1,Segment s2){
    return equals(dot(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

bool isParallel(Vector a,Vector b){
    return equals(cross(a,b),0.0);
}

bool isParallel(Point a1,Point a2,Point b1,Point b2){
    return isParallel(a1-a2,b1-b2);
}

bool isParallel(Segment s1,Segment s2){
    return equals(cross(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

Point project(Segment s,Point p){
    Vector base=s.p2-s.p1;
    double r=dot(p-s.p1,base)/norm(base);
    return s.p1+base*r;
}

Point reflect(Segment s,Point p){
    return p+(project(s,p)-p)*2.0;
}

//p0,p1,p2の順に見たときどうなるか？

static const int counter_clockwise=1;
static const int clockwise=-1;
static const int online_back=2;
static const int online_front=-2;
static const int on_segment=0;

int ccw(Point p0,Point p1,Point p2){
    Vector a=p1-p0;
    Vector b=p2-p0;
    
    if(cross(a,b)>eps) return counter_clockwise;
    if(cross(a,b)<-eps) return clockwise;
    if(dot(a,b)<-eps) return online_back;
    if(a.norm()<b.norm()) return online_front;
    
    return on_segment;
}

bool intersect(Point p1,Point p2,Point p3,Point p4){
    return(ccw(p1,p2,p3)*ccw(p1,p2,p4)<=0&&ccw(p3,p4,p1)*ccw(p3,p4,p2)<=0);
}

bool intersect(Segment s1,Segment s2){
    return intersect(s1.p1,s1.p2,s2.p1,s2.p2);
}

bool overlap(Segment s1,Segment s2){
    int a=ccw(s1.p1,s1.p2,s2.p1),b=ccw(s1.p1,s1.p2,s2.p2);
    if(a&1||b&1) return 0;
    if(a==2){
        if(b==-2||(b==0&&!(s2.p2==s1.p1))) return 1;
        else return 0;
    }
    if(a==-2){
        if(b==2||(b==0&&!(s2.p2==s1.p2))) return 1;
        else return 0;
    }
    if(a==0){
        if(s1.p1==s2.p1){
            if(b!=2) return 1;
            else return 0;
        }
        else if(s1.p2==s2.p1){
            if(b!=-2) return 1;
            else return 0;
        }
        else return 1;
    }
    return 0;
}
//s1とs2の共通の線分(長さ0より大きい)があるかどうか

typedef Segment Line;

double getDistance(Point a,Point b){
    return abs(a-b);
}

double getDistanceLP(Line l,Point p){
    return abs(cross(l.p2-l.p1,p-l.p1)/abs(l.p2-l.p1));
}

double getDistanceSP(Segment s,Point p){
    if(dot(s.p2-s.p1,p-s.p1)<0.0) return abs(p-s.p1);
    if(dot(s.p1-s.p2,p-s.p2)<0.0) return abs(p-s.p2);
    return getDistanceLP(s,p);
}

double getDistance(Segment s1,Segment s2){
    if(intersect(s1,s2)) return 0.0;
    return min({getDistanceSP(s1,s2.p1),getDistanceSP(s1,s2.p2),getDistanceSP(s2,s1.p1),getDistanceSP(s2,s1.p2)});
}

Point getCrossPointS(Segment s1,Segment s2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    Vector base=s2.p2-s2.p1;
    double d1=abs(cross(base,s1.p1-s2.p1));
    double d2=abs(cross(base,s1.p2-s2.p1));
    double t=d1/(d1+d2);
    return s1.p1+(s1.p2-s1.p1)*t;
}//同じ時壊れます

Point getCrossPointL(Line l1,Line l2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    
    Vector v1=l1.p2-l1.p1,v2=l2.p2-l2.p1;
    
    return l1.p1+v1*cross(v2,l2.p1-l1.p1)/cross(v2,v1);
}

Segment ParallelSegment(Segment s,double d){
    Vector v={-(s.p2-s.p1).y,(s.p2-s.p1).x};
    v=v/abs(v);
    
    s.p1=s.p1+v*d;
    s.p2=s.p2+v*d;
    
    return s;
}

Point naisin(Point p1,Point p2,Point p3){
    if(p1==p2&&p2==p3&&p3==p1) return p1;
    
    return (p1*abs(p2-p3)+p2*abs(p1-p3)+p3*abs(p1-p2))/(abs(p2-p3)+abs(p1-p3)+abs(p1-p2));
}

Point naisin(Line l1,Line l2,Line l3){
    //平行でない前提
    
    Point p1=getCrossPointL(l1,l2),p2=getCrossPointL(l1,l3),p3=getCrossPointL(l2,l3);
    return naisin(p1,p2,p3);
}

//ネットの適当を書いたのであってるか全く知りません→あってそう

class Circle{
public:
    Point c;
    double r;
    Circle(Point c=Point(),double r=0.0):c(c),r(r){}
};

Point CircleCenter(Point a,Point b,Point c){
    Point u=a-b,v=a-c;
    double m1=(norm(a)-norm(b))/2.0,m2=(norm(a)-norm(c))/2.0;
    
    Point res;
    if(cross(u,v)==0.0){
        res.x=1e9;
        res.y=1e9;
        
        return res;
    }
    res.x=(m1*v.y-m2*u.y)/cross(u,v);
    res.y=(m1*v.x-m2*u.x)/cross(v,u);
    
    return res;
}
//3点を通る円の中心を返す

//交わる 0
// c1がc2のinside 1
// c1がc2のoutside 2
// 交わらない 3

int not_intersect(Circle c1,Circle c2){
    double d=getDistance(c1.c,c2.c);
    double r1=c1.r,r2=c2.r;
    if(r1<r2){
        if(d<(r2-r1)) return 1;
    }
    if(r1>r2){
        if(d<(r1-r2)) return 2;
    }
    if(d<=r1+r2) return 0;
    else return 3;
}

pair<Point,Point> segCrossPpoints(Circle c,Line l){
    //assert(intersect(c,l));
    Vector pr=project(l,c.c);
    Vector e=(l.p2-l.p1)/abs(l.p2-l.p1);
    double base=sqrt(c.r*c.r-norm(pr-c.c));
    return make_pair(pr+e*base,pr-e*base);
}

double arg(Vector p){return atan2(p.y,p.x);}
Vector polar(double a,double r){return Point(cos(r)*a,sin(r)*a);}

//inside(outside)

pair<Point,Point> getCrossPoints(Circle c1,Circle c2){
    //assert(intersect(c1,c2));
    double d=abs(c1.c-c2.c);
    double a=acos((c1.r*c1.r+d*d-c2.r*c2.r)/(2*c1.r*d));
    double t=arg(c2.c-c1.c);
    return make_pair(c1.c+polar(c1.r,t+a),c1.c+polar(c1.r,t-a));
}

vector<Line> Commontangent(Circle c1,Circle c2){
    vector<Line> res;
    Point p=c2.c-c1.c;
    
    if(abs(p)>=(c1.r+c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r+c2.r)+p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r+c2.r)-p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r+c2.r)-p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r+c2.r)+p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    if(abs(p)>=abs(c1.r-c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r-c2.r)+p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r-c2.r)-p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r-c2.r)-p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r-c2.r)+p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    return res;
}

typedef vector<Point> Polygon;

/*
 IN 2
 ON 1
 OUT 0
 */

int contains(Polygon g,Point p){
    int n=int(g.size());
    bool x=false;
    for(int i=0;i<n;i++){
        Point a=g[i]-p,b=g[(i+1)%n]-p;
        if(a.y>b.y) swap(a,b);
        if(a.y<=0&&0<b.y&&cross(a,b)<0) x=!x;
        if(abs(cross(a,b))<=0&&dot(a,b)<=0) return 1;
    }
    return (x?2:0);
}

Polygon andrewScan(Polygon s,bool ok){
    Polygon u,l;
    sort(all(s));
    s.erase(unique(all(s)),s.end());
    
    if(int(s.size())<3) return s;
    int n=int(s.size());
    
    u.push_back(s[0]);
    u.push_back(s[1]);
    
    l.push_back(s[n-1]);
    l.push_back(s[n-2]);
    
    if(ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])==counter_clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])==counter_clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    if(!ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])!=clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])!=clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    reverse(all(l));
    
    for(int i=int(u.size())-2;i>=1;i--) l.push_back(u[i]);
    
    return l;
}//ok==1なら辺の上も含める

Polygon convex_cut(const Polygon& P, const Line& l) {
    Polygon Q;
    for(int i=0;i<si(P);i++){
        Point A=P[i],B=P[(i+1)%si(P)];
        if(ccw(l.p1,l.p2,A)!=-1)Q.push_back(A);
        if(ccw(l.p1,l.p2,A)*ccw(l.p1,l.p2,B)<0) Q.push_back(getCrossPointL(Line{A,B},l));
    }
    return Q;
}

double area(Point a,Point b,Point c){
    b=b-a;
    c=c-a;
    return abs(b.x*c.y-b.y*c.x)/2.0;
}

ll gcd(ll a,ll b){
    if(b==0) return a;
    return gcd(b,a%b);
}

pair<Point,Vector> perpendicular_bisector(Point a,Point b){
    Point c=(a+b)/2;
    Vector v=b-c;
    swap(v.x,v.y);
    v.x*=-1;
    
    Point p=c;
    if(v.x==0){
        v.y=1;
        p.y=0;
    }
    else if(v.y==0){
        v.x=1;
        p.x=0;
    }
    else{
        if(v.x<0){
            v.x*=-1;
            v.y*=-1;
        }
        ll g=gcd(abs(ll(v.x)),abs(ll(v.y)));
        v.x/=g;
        v.y/=g;
        if(p.x>=0){
            ll d=p.x/v.x;
            p=p-v*d;
        }else{
            ll d=abs(p.x)/v.x;
            p=p+v*d;
            
            if(p.x<0){
                p=p+v;
            }
        }
    }
    
    return mp(p,v);
}
//2倍するなりして整数にしておくこと

int main() {
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int N;cin>>N;
    vector<Point> P(N);
    for(int i=0;i<N;i++) cin>>P[i].x>>P[i].y;
    bool f=false;
    for(int q=0;q<2;q++){
        bool ok=true;
        for(int i=2;i+1<N;i++){
            Polygon Q;
            for(int j=0;j<=i;j++) Q.push_back(P[j]);
            Q=andrewScan(Q,0);
            Point PP=P[i];
            if(contains(Q,PP)==2) ok=false;
        }
        for(int i=0;i+1<N-2;i++){
            Polygon Q;
            for(int j=i+1;j<N;j++) Q.push_back(P[j]);
            Q=andrewScan(Q,0);
            Point PP=P[i+1];
            if(contains(Q,PP)==2) ok=false;
        }
        if(ok) f=true;
        
        reverse(all(P));
    }
    if(f) cout<<"Possible\n";
    else cout<<"Impossible\n";
}
accuracy: 0.007352941176470588
time_ms: 40
memory_kb: 3596
final_rank: 18

code_snippet: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=305,INF=1<<29;
#define double ll

//幾何ライブラリ
// define double ll をするときは Point の < と == も書き換えよう！

const double eps=1e-8;
const double pi=acos((double)-1.0L);
#define equals(a,b) (fabs((a)-(b))<eps)

double torad(double deg) {return (double)(deg)*pi/180.0;}
double todeg(double ang) {return ang*180.0/pi;}

class Point{
public:
    double x,y;
    
    Point(double x=0,double y=0):x(x),y(y){}
    
    Point operator + (Point p){return Point(x+p.x,y+p.y);}
    Point operator - (Point p){return Point(x-p.x,y-p.y);}
    Point operator * (double a){return Point(a*x,a*y);}
    Point operator / (double a){return Point(x/a,y/a);}
    
    double abs(){return sqrt(norm());}
    double norm(){return x*x+y*y;}
    
    bool operator < (const Point &p)const{
        return x<p.x||(x==p.x&&y<p.y);
    }
    
    bool operator == (const Point &p)const{
        return x==p.x&&y==p.y;
    }
};

typedef Point Vector;

double norm(Vector a){
    return a.x*a.x+a.y*a.y;
}

double abs(Vector a){
    return sqrt(norm(a));
}

double dot(Vector a,Vector b){
    return a.x*b.x+a.y*b.y;
}

double cross(Vector a,Vector b){
    return a.x*b.y-a.y*b.x;
}

struct Segment{
    Point p1,p2;
};

bool isOrthogonal(Vector a,Vector b){
    return equals(dot(a,b),0.0);
}

bool isOrthogonal(Point a1,Point a2,Point b1,Point b2){
    return isOrthogonal(a1-a2,b1-b2);
}

bool isOrthogonal(Segment s1,Segment s2){
    return equals(dot(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

bool isParallel(Vector a,Vector b){
    return equals(cross(a,b),0.0);
}

bool isParallel(Point a1,Point a2,Point b1,Point b2){
    return isParallel(a1-a2,b1-b2);
}

bool isParallel(Segment s1,Segment s2){
    return equals(cross(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

Point project(Segment s,Point p){
    Vector base=s.p2-s.p1;
    double r=dot(p-s.p1,base)/norm(base);
    return s.p1+base*r;
}

Point reflect(Segment s,Point p){
    return p+(project(s,p)-p)*2.0;
}

//p0,p1,p2の順に見たときどうなるか？

static const int counter_clockwise=1;
static const int clockwise=-1;
static const int online_back=2;
static const int online_front=-2;
static const int on_segment=0;

int ccw(Point p0,Point p1,Point p2){
    Vector a=p1-p0;
    Vector b=p2-p0;
    
    if(cross(a,b)>eps) return counter_clockwise;
    if(cross(a,b)<-eps) return clockwise;
    if(dot(a,b)<-eps) return online_back;
    if(a.norm()<b.norm()) return online_front;
    
    return on_segment;
}

bool intersect(Point p1,Point p2,Point p3,Point p4){
    return(ccw(p1,p2,p3)*ccw(p1,p2,p4)<=0&&ccw(p3,p4,p1)*ccw(p3,p4,p2)<=0);
}

bool intersect(Segment s1,Segment s2){
    return intersect(s1.p1,s1.p2,s2.p1,s2.p2);
}

bool overlap(Segment s1,Segment s2){
    int a=ccw(s1.p1,s1.p2,s2.p1),b=ccw(s1.p1,s1.p2,s2.p2);
    if(a&1||b&1) return 0;
    if(a==2){
        if(b==-2||(b==0&&!(s2.p2==s1.p1))) return 1;
        else return 0;
    }
    if(a==-2){
        if(b==2||(b==0&&!(s2.p2==s1.p2))) return 1;
        else return 0;
    }
    if(a==0){
        if(s1.p1==s2.p1){
            if(b!=2) return 1;
            else return 0;
        }
        else if(s1.p2==s2.p1){
            if(b!=-2) return 1;
            else return 0;
        }
        else return 1;
    }
    return 0;
}
//s1とs2の共通の線分(長さ0より大きい)があるかどうか

typedef Segment Line;

double getDistance(Point a,Point b){
    return abs(a-b);
}

double getDistanceLP(Line l,Point p){
    return abs(cross(l.p2-l.p1,p-l.p1)/abs(l.p2-l.p1));
}

double getDistanceSP(Segment s,Point p){
    if(dot(s.p2-s.p1,p-s.p1)<0.0) return abs(p-s.p1);
    if(dot(s.p1-s.p2,p-s.p2)<0.0) return abs(p-s.p2);
    return getDistanceLP(s,p);
}

double getDistance(Segment s1,Segment s2){
    if(intersect(s1,s2)) return 0.0;
    return min({getDistanceSP(s1,s2.p1),getDistanceSP(s1,s2.p2),getDistanceSP(s2,s1.p1),getDistanceSP(s2,s1.p2)});
}

Point getCrossPointS(Segment s1,Segment s2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    Vector base=s2.p2-s2.p1;
    double d1=abs(cross(base,s1.p1-s2.p1));
    double d2=abs(cross(base,s1.p2-s2.p1));
    double t=d1/(d1+d2);
    return s1.p1+(s1.p2-s1.p1)*t;
}//同じ時壊れます

Point getCrossPointL(Line l1,Line l2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    
    Vector v1=l1.p2-l1.p1,v2=l2.p2-l2.p1;
    
    return l1.p1+v1*cross(v2,l2.p1-l1.p1)/cross(v2,v1);
}

Segment ParallelSegment(Segment s,double d){
    Vector v={-(s.p2-s.p1).y,(s.p2-s.p1).x};
    v=v/abs(v);
    
    s.p1=s.p1+v*d;
    s.p2=s.p2+v*d;
    
    return s;
}

Point naisin(Point p1,Point p2,Point p3){
    if(p1==p2&&p2==p3&&p3==p1) return p1;
    
    return (p1*abs(p2-p3)+p2*abs(p1-p3)+p3*abs(p1-p2))/(abs(p2-p3)+abs(p1-p3)+abs(p1-p2));
}

Point naisin(Line l1,Line l2,Line l3){
    //平行でない前提
    
    Point p1=getCrossPointL(l1,l2),p2=getCrossPointL(l1,l3),p3=getCrossPointL(l2,l3);
    return naisin(p1,p2,p3);
}

//ネットの適当を書いたのであってるか全く知りません→あってそう

class Circle{
public:
    Point c;
    double r;
    Circle(Point c=Point(),double r=0.0):c(c),r(r){}
};

Point CircleCenter(Point a,Point b,Point c){
    Point u=a-b,v=a-c;
    double m1=(norm(a)-norm(b))/2.0,m2=(norm(a)-norm(c))/2.0;
    
    Point res;
    if(cross(u,v)==0.0){
        res.x=1e9;
        res.y=1e9;
        
        return res;
    }
    res.x=(m1*v.y-m2*u.y)/cross(u,v);
    res.y=(m1*v.x-m2*u.x)/cross(v,u);
    
    return res;
}
//3点を通る円の中心を返す

//交わる 0
// c1がc2のinside 1
// c1がc2のoutside 2
// 交わらない 3

int not_intersect(Circle c1,Circle c2){
    double d=getDistance(c1.c,c2.c);
    double r1=c1.r,r2=c2.r;
    if(r1<r2){
        if(d<(r2-r1)) return 1;
    }
    if(r1>r2){
        if(d<(r1-r2)) return 2;
    }
    if(d<=r1+r2) return 0;
    else return 3;
}

pair<Point,Point> segCrossPpoints(Circle c,Line l){
    //assert(intersect(c,l));
    Vector pr=project(l,c.c);
    Vector e=(l.p2-l.p1)/abs(l.p2-l.p1);
    double base=sqrt(c.r*c.r-norm(pr-c.c));
    return make_pair(pr+e*base,pr-e*base);
}

double arg(Vector p){return atan2(p.y,p.x);}
Vector polar(double a,double r){return Point(cos(r)*a,sin(r)*a);}

//inside(outside)

pair<Point,Point> getCrossPoints(Circle c1,Circle c2){
    //assert(intersect(c1,c2));
    double d=abs(c1.c-c2.c);
    double a=acos((c1.r*c1.r+d*d-c2.r*c2.r)/(2*c1.r*d));
    double t=arg(c2.c-c1.c);
    return make_pair(c1.c+polar(c1.r,t+a),c1.c+polar(c1.r,t-a));
}

vector<Line> Commontangent(Circle c1,Circle c2){
    vector<Line> res;
    Point p=c2.c-c1.c;
    
    if(abs(p)>=(c1.r+c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r+c2.r)+p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r+c2.r)-p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r+c2.r)-p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r+c2.r)+p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    if(abs(p)>=abs(c1.r-c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r-c2.r)+p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r-c2.r)-p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r-c2.r)-p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r-c2.r)+p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    return res;
}

typedef vector<Point> Polygon;

/*
 IN 2
 ON 1
 OUT 0
 */

int contains(Polygon g,Point p){
    int n=int(g.size());
    bool x=false;
    for(int i=0;i<n;i++){
        Point a=g[i]-p,b=g[(i+1)%n]-p;
        if(a.y>b.y) swap(a,b);
        if(a.y<eps&&0<b.y&&cross(a,b)<0) x=!x;
        if(abs(cross(a,b))<eps&&dot(a,b)<eps) return 1;
    }
    return (x?2:0);
}

Polygon andrewScan(Polygon s,bool ok){
    Polygon u,l;
    sort(all(s));
    
    if(int(s.size())<3) return s;
    int n=int(s.size());
    
    u.push_back(s[0]);
    u.push_back(s[1]);
    
    l.push_back(s[n-1]);
    l.push_back(s[n-2]);
    
    if(ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])==counter_clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])==counter_clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    if(!ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])!=clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])!=clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    reverse(all(l));
    
    for(int i=int(u.size())-2;i>=1;i--) l.push_back(u[i]);
    
    return l;
}//ok==1なら辺の上も含める

Polygon convex_cut(const Polygon& P, const Line& l) {
    Polygon Q;
    for(int i=0;i<si(P);i++){
        Point A=P[i],B=P[(i+1)%si(P)];
        if(ccw(l.p1,l.p2,A)!=-1)Q.push_back(A);
        if(ccw(l.p1,l.p2,A)*ccw(l.p1,l.p2,B)<0) Q.push_back(getCrossPointL(Line{A,B},l));
    }
    return Q;
}

double area(Point a,Point b,Point c){
    b=b-a;
    c=c-a;
    return abs(b.x*c.y-b.y*c.x)/2.0;
}

ll gcd(ll a,ll b){
    if(b==0) return a;
    return gcd(b,a%b);
}

pair<Point,Vector> perpendicular_bisector(Point a,Point b){
    Point c=(a+b)/2;
    Vector v=b-c;
    swap(v.x,v.y);
    v.x*=-1;
    
    Point p=c;
    if(v.x==0){
        v.y=1;
        p.y=0;
    }
    else if(v.y==0){
        v.x=1;
        p.x=0;
    }
    else{
        if(v.x<0){
            v.x*=-1;
            v.y*=-1;
        }
        ll g=gcd(abs(ll(v.x)),abs(ll(v.y)));
        v.x/=g;
        v.y/=g;
        if(p.x>=0){
            ll d=p.x/v.x;
            p=p-v*d;
        }else{
            ll d=abs(p.x)/v.x;
            p=p+v*d;
            
            if(p.x<0){
                p=p+v;
            }
        }
    }
    
    return mp(p,v);
}
//2倍するなりして整数にしておくこと

int main() {
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int N;cin>>N;
    vector<Point> P(N);
    for(int i=0;i<N;i++) cin>>P[i].x>>P[i].y;
    bool f=false;
    for(int q=0;q<2;q++){
        bool ok=true;
        for(int i=2;i+1<N;i++){
            Polygon Q;
            for(int j=0;j<=i;j++) Q.push_back(P[j]);
            Q=andrewScan(Q,0);
            Point PP=P[i];
            if(contains(Q,PP)==2) ok=false;
        }
        for(int i=0;i+1<N-2;i++){
            Polygon Q;
            for(int j=i+1;j<N;j++) Q.push_back(P[j]);
            Q=andrewScan(Q,0);
            Point PP=P[i+1];
            if(contains(Q,PP)==2) ok=false;
        }
        if(ok) f=true;
        
        reverse(all(P));
    }
    if(f) cout<<"Possible\n";
    else cout<<"Impossible\n";
}
accuracy: 0.014705882352941176
time_ms: 40
memory_kb: 3508
final_rank: 14

code_snippet: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=305,INF=1<<29;

//幾何ライブラリ
// define double ll をするときは Point の < と == も書き換えよう！

const double eps=1e-8;
const double pi=acos((double)-1.0L);
#define equals(a,b) (fabs((a)-(b))<eps)

double torad(double deg) {return (double)(deg)*pi/180.0;}
double todeg(double ang) {return ang*180.0/pi;}

class Point{
public:
    double x,y;
    
    Point(double x=0,double y=0):x(x),y(y){}
    
    Point operator + (Point p){return Point(x+p.x,y+p.y);}
    Point operator - (Point p){return Point(x-p.x,y-p.y);}
    Point operator * (double a){return Point(a*x,a*y);}
    Point operator / (double a){return Point(x/a,y/a);}
    
    double abs(){return sqrt(norm());}
    double norm(){return x*x+y*y;}
    
    bool operator < (const Point &p)const{
        return x+eps<p.x||(equals(x,p.x)&&y+eps<p.y);
    }
    
    bool operator == (const Point &p)const{
        return fabs(x-p.x)<eps/100000&&fabs(y-p.y)<eps/100000;
    }
};

typedef Point Vector;

double norm(Vector a){
    return a.x*a.x+a.y*a.y;
}

double abs(Vector a){
    return sqrt(norm(a));
}

double dot(Vector a,Vector b){
    return a.x*b.x+a.y*b.y;
}

double cross(Vector a,Vector b){
    return a.x*b.y-a.y*b.x;
}

struct Segment{
    Point p1,p2;
};

bool isOrthogonal(Vector a,Vector b){
    return equals(dot(a,b),0.0);
}

bool isOrthogonal(Point a1,Point a2,Point b1,Point b2){
    return isOrthogonal(a1-a2,b1-b2);
}

bool isOrthogonal(Segment s1,Segment s2){
    return equals(dot(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

bool isParallel(Vector a,Vector b){
    return equals(cross(a,b),0.0);
}

bool isParallel(Point a1,Point a2,Point b1,Point b2){
    return isParallel(a1-a2,b1-b2);
}

bool isParallel(Segment s1,Segment s2){
    return equals(cross(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

Point project(Segment s,Point p){
    Vector base=s.p2-s.p1;
    double r=dot(p-s.p1,base)/norm(base);
    return s.p1+base*r;
}

Point reflect(Segment s,Point p){
    return p+(project(s,p)-p)*2.0;
}

Point turn(Point p,Point c,double pi){
    double q=atan2(p.y-c.y,p.x-c.x);
    q+=pi;
    p=c+Point{cos(q)*abs(p-c),sin(q)*abs(p-c)};
    
    return p;
}
//pをcを中心としてpi回転させる(1周で2π)
//p=cのときnan

//p0,p1,p2の順に見たときどうなるか？

static const int counter_clockwise=1;
static const int clockwise=-1;
static const int online_back=2;
static const int online_front=-2;
static const int on_segment=0;

int ccw(Point p0,Point p1,Point p2){
    Vector a=p1-p0;
    Vector b=p2-p0;
    
    if(cross(a,b)>eps) return counter_clockwise;
    if(cross(a,b)<-eps) return clockwise;
    if(dot(a,b)<-eps) return online_back;
    if(a.norm()<b.norm()) return online_front;
    
    return on_segment;
}

bool intersect(Point p1,Point p2,Point p3,Point p4){
    return(ccw(p1,p2,p3)*ccw(p1,p2,p4)<=0&&ccw(p3,p4,p1)*ccw(p3,p4,p2)<=0);
}

bool intersect(Segment s1,Segment s2){
    return intersect(s1.p1,s1.p2,s2.p1,s2.p2);
}

bool overlap(Segment s1,Segment s2){
    int a=ccw(s1.p1,s1.p2,s2.p1),b=ccw(s1.p1,s1.p2,s2.p2);
    if(a&1||b&1) return 0;
    if(a==2){
        if(b==-2||(b==0&&!(s2.p2==s1.p1))) return 1;
        else return 0;
    }
    if(a==-2){
        if(b==2||(b==0&&!(s2.p2==s1.p2))) return 1;
        else return 0;
    }
    if(a==0){
        if(s1.p1==s2.p1){
            if(b!=2) return 1;
            else return 0;
        }
        else if(s1.p2==s2.p1){
            if(b!=-2) return 1;
            else return 0;
        }
        else return 1;
    }
    return 0;
}
//s1とs2の共通の線分(長さ0より大きい)があるかどうか

typedef Segment Line;

double getDistance(Point a,Point b){
    return abs(a-b);
}

double getDistanceLP(Line l,Point p){
    return abs(cross(l.p2-l.p1,p-l.p1)/abs(l.p2-l.p1));
}

double getDistanceSP(Segment s,Point p){
    if(dot(s.p2-s.p1,p-s.p1)<0.0) return abs(p-s.p1);
    if(dot(s.p1-s.p2,p-s.p2)<0.0) return abs(p-s.p2);
    return getDistanceLP(s,p);
}

double getDistance(Segment s1,Segment s2){
    if(intersect(s1,s2)) return 0.0;
    return min({getDistanceSP(s1,s2.p1),getDistanceSP(s1,s2.p2),getDistanceSP(s2,s1.p1),getDistanceSP(s2,s1.p2)});
}

Point getCrossPointS(Segment s1,Segment s2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    Vector base=s2.p2-s2.p1;
    double d1=abs(cross(base,s1.p1-s2.p1));
    double d2=abs(cross(base,s1.p2-s2.p1));
    double t=d1/(d1+d2);
    return s1.p1+(s1.p2-s1.p1)*t;
}//同じ時壊れます

Point getCrossPointL(Line l1,Line l2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    
    Vector v1=l1.p2-l1.p1,v2=l2.p2-l2.p1;
    
    return l1.p1+v1*cross(v2,l2.p1-l1.p1)/cross(v2,v1);
}

Segment ParallelSegment(Segment s,double d){
    Vector v={-(s.p2-s.p1).y,(s.p2-s.p1).x};
    v=v/abs(v);
    
    s.p1=s.p1+v*d;
    s.p2=s.p2+v*d;
    
    return s;
}

Point naisin(Point p1,Point p2,Point p3){
    if(p1==p2&&p2==p3&&p3==p1) return p1;
    
    return (p1*abs(p2-p3)+p2*abs(p1-p3)+p3*abs(p1-p2))/(abs(p2-p3)+abs(p1-p3)+abs(p1-p2));
}

Point naisin(Line l1,Line l2,Line l3){
    //平行でない前提
    
    Point p1=getCrossPointL(l1,l2),p2=getCrossPointL(l1,l3),p3=getCrossPointL(l2,l3);
    return naisin(p1,p2,p3);
}

//ネットの適当を書いたのであってるか全く知りません→あってそう

class Circle{
public:
    Point c;
    double r;
    Circle(Point c=Point(),double r=0.0):c(c),r(r){}
};

Point CircleCenter(Point a,Point b,Point c){
    Point u=a-b,v=a-c;
    double m1=(norm(a)-norm(b))/2.0,m2=(norm(a)-norm(c))/2.0;
    
    Point res;
    if(cross(u,v)==0.0){
        res.x=1e9;
        res.y=1e9;
        
        return res;
    }
    res.x=(m1*v.y-m2*u.y)/cross(u,v);
    res.y=(m1*v.x-m2*u.x)/cross(v,u);
    
    return res;
}
//3点を通る円の中心を返す

//交わる 0
// c1がc2のinside 1
// c1がc2のoutside 2
// 交わらない 3

int not_intersect(Circle c1,Circle c2){
    double d=getDistance(c1.c,c2.c);
    double r1=c1.r,r2=c2.r;
    if(r1<r2){
        if(d<(r2-r1)) return 1;
    }
    if(r1>r2){
        if(d<(r1-r2)) return 2;
    }
    if(d<=r1+r2) return 0;
    else return 3;
}

pair<Point,Point> segCrossPpoints(Circle c,Line l){
    //assert(intersect(c,l));
    Vector pr=project(l,c.c);
    Vector e=(l.p2-l.p1)/abs(l.p2-l.p1);
    double base=sqrt(c.r*c.r-norm(pr-c.c));
    return make_pair(pr+e*base,pr-e*base);
}

double arg(Vector p){return atan2(p.y,p.x);}
Vector polar(double a,double r){return Point(cos(r)*a,sin(r)*a);}

//inside(outside)

pair<Point,Point> getCrossPoints(Circle c1,Circle c2){
    //assert(intersect(c1,c2));
    double d=abs(c1.c-c2.c);
    double a=acos((c1.r*c1.r+d*d-c2.r*c2.r)/(2*c1.r*d));
    double t=arg(c2.c-c1.c);
    return make_pair(c1.c+polar(c1.r,t+a),c1.c+polar(c1.r,t-a));
}

vector<Line> Commontangent(Circle c1,Circle c2){
    vector<Line> res;
    Point p=c2.c-c1.c;
    
    if(abs(p)>=(c1.r+c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r+c2.r)+p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r+c2.r)-p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r+c2.r)-p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r+c2.r)+p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    if(abs(p)>=abs(c1.r-c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r-c2.r)+p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r-c2.r)-p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r-c2.r)-p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r-c2.r)+p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    return res;
}

typedef vector<Point> Polygon;

/*
 IN 2
 ON 1
 OUT 0
 */

int contains(Polygon g,Point p){
    int n=int(g.size());
    bool x=false;
    for(int i=0;i<n;i++){
        Point a=g[i]-p,b=g[(i+1)%n]-p;
        if(a.y>b.y) swap(a,b);
        if(a.y<eps&&0<b.y&&cross(a,b)<0) x=!x;
        if(abs(cross(a,b))<eps&&dot(a,b)<eps) return 1;
    }
    return (x?2:0);
}

Polygon andrewScan(Polygon s,bool ok){
    Polygon u,l;
    sort(all(s));
    
    if(int(s.size())<3) return s;
    int n=int(s.size());
    
    u.push_back(s[0]);
    u.push_back(s[1]);
    
    l.push_back(s[n-1]);
    l.push_back(s[n-2]);
    
    if(ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])==counter_clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])==counter_clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    if(!ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])!=clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])!=clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    reverse(all(l));
    
    for(int i=int(u.size())-2;i>=1;i--) l.push_back(u[i]);
    
    return l;
}//ok==1なら辺の上も含める

Polygon convex_cut(const Polygon& P, const Line& l) {
    Polygon Q;
    for(int i=0;i<si(P);i++){
        Point A=P[i],B=P[(i+1)%si(P)];
        if(ccw(l.p1,l.p2,A)!=-1)Q.push_back(A);
        if(ccw(l.p1,l.p2,A)*ccw(l.p1,l.p2,B)<0) Q.push_back(getCrossPointL(Line{A,B},l));
    }
    return Q;
}

double area(Point a,Point b,Point c){
    b=b-a;
    c=c-a;
    return abs(b.x*c.y-b.y*c.x)/2.0;
}

double area(Polygon &P){
    if(si(P)==0) return 0.0;
    double res=0;
    Point c={0.0,0.0};
    for(int i=0;i<si(P);i++){
        c=c+P[i];
    }
    c=c/si(P);
    
    for(int i=0;i<si(P);i++){
        res+=area(c,P[i],P[(i+1)%si(P)]);
    }
    
    return res;
}

ll gcd(ll a,ll b){
    if(b==0) return a;
    return gcd(b,a%b);
}

pair<Point,Vector> perpendicular_bisector(Point a,Point b){
    Point c=(a+b)/2;
    Vector v=b-c;
    swap(v.x,v.y);
    v.x*=-1;
    
    Point p=c;
    if(v.x==0){
        v.y=1;
        p.y=0;
    }
    else if(v.y==0){
        v.x=1;
        p.x=0;
    }
    else{
        if(v.x<0){
            v.x*=-1;
            v.y*=-1;
        }
        ll g=gcd(abs(ll(v.x)),abs(ll(v.y)));
        v.x/=g;
        v.y/=g;
        if(p.x>=0){
            ll d=p.x/v.x;
            p=p-v*d;
        }else{
            ll d=abs(p.x)/v.x;
            p=p+v*d;
            
            if(p.x<0){
                p=p+v;
            }
        }
    }
    
    return mp(p,v);
}
//2倍するなりして整数にしておくこと

int main() {
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int N;cin>>N;
    vector<Point> P(N);
    for(int i=0;i<N;i++) cin>>P[i].x>>P[i].y;
    bool f=false;
    for(int q=0;q<2;q++){
        bool ok=true;
        for(int i=2;i+1<N;i++){
            Polygon Q;
            for(int j=0;j<=i;j++) Q.push_back(P[j]);
            Q=andrewScan(Q,0);
            Point PP=P[i]+(P[i+1]-P[i])/abs(P[i+1]-P[i])/1e8;
            if(contains(Q,PP)==2) ok=false;
        }
        for(int i=0;i+1<N-2;i++){
            Polygon Q;
            for(int j=i+1;j<N;j++) Q.push_back(P[j]);
            Q=andrewScan(Q,0);
            Point PP=P[i+1]+(P[i]-P[i+1])/abs(P[i]-P[i+1])/1e8;
            if(contains(Q,PP)==2) ok=false;
        }
        if(ok) f=true;
        
        reverse(all(P));
    }
    if(f) cout<<"Possible\n";
    else cout<<"Impossible\n";
}
accuracy: 0.007352941176470588
time_ms: 50
memory_kb: 3620
final_rank: 19

code_snippet: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=305,INF=1<<29;

//幾何ライブラリ
// define double ll をするときは Point の < と == も書き換えよう！

const double eps=1e-8;
const double pi=acos((double)-1.0L);
#define equals(a,b) (fabs((a)-(b))<eps)

double torad(double deg) {return (double)(deg)*pi/180.0;}
double todeg(double ang) {return ang*180.0/pi;}

class Point{
public:
    double x,y;
    
    Point(double x=0,double y=0):x(x),y(y){}
    
    Point operator + (Point p){return Point(x+p.x,y+p.y);}
    Point operator - (Point p){return Point(x-p.x,y-p.y);}
    Point operator * (double a){return Point(a*x,a*y);}
    Point operator / (double a){return Point(x/a,y/a);}
    
    double abs(){return sqrt(norm());}
    double norm(){return x*x+y*y;}
    
    bool operator < (const Point &p)const{
        return x+eps<p.x||(equals(x,p.x)&&y+eps<p.y);
    }
    
    bool operator == (const Point &p)const{
        return fabs(x-p.x)<eps/100000&&fabs(y-p.y)<eps/100000;
    }
};

typedef Point Vector;

double norm(Vector a){
    return a.x*a.x+a.y*a.y;
}

double abs(Vector a){
    return sqrt(norm(a));
}

double dot(Vector a,Vector b){
    return a.x*b.x+a.y*b.y;
}

double cross(Vector a,Vector b){
    return a.x*b.y-a.y*b.x;
}

struct Segment{
    Point p1,p2;
};

bool isOrthogonal(Vector a,Vector b){
    return equals(dot(a,b),0.0);
}

bool isOrthogonal(Point a1,Point a2,Point b1,Point b2){
    return isOrthogonal(a1-a2,b1-b2);
}

bool isOrthogonal(Segment s1,Segment s2){
    return equals(dot(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

bool isParallel(Vector a,Vector b){
    return equals(cross(a,b),0.0);
}

bool isParallel(Point a1,Point a2,Point b1,Point b2){
    return isParallel(a1-a2,b1-b2);
}

bool isParallel(Segment s1,Segment s2){
    return equals(cross(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

Point project(Segment s,Point p){
    Vector base=s.p2-s.p1;
    double r=dot(p-s.p1,base)/norm(base);
    return s.p1+base*r;
}

Point reflect(Segment s,Point p){
    return p+(project(s,p)-p)*2.0;
}

Point turn(Point p,Point c,double pi){
    double q=atan2(p.y-c.y,p.x-c.x);
    q+=pi;
    p=c+Point{cos(q)*abs(p-c),sin(q)*abs(p-c)};
    
    return p;
}
//pをcを中心としてpi回転させる(1周で2π)
//p=cのときnan

//p0,p1,p2の順に見たときどうなるか？

static const int counter_clockwise=1;
static const int clockwise=-1;
static const int online_back=2;
static const int online_front=-2;
static const int on_segment=0;

int ccw(Point p0,Point p1,Point p2){
    Vector a=p1-p0;
    Vector b=p2-p0;
    
    if(cross(a,b)>eps) return counter_clockwise;
    if(cross(a,b)<-eps) return clockwise;
    if(dot(a,b)<-eps) return online_back;
    if(a.norm()<b.norm()) return online_front;
    
    return on_segment;
}

bool intersect(Point p1,Point p2,Point p3,Point p4){
    return(ccw(p1,p2,p3)*ccw(p1,p2,p4)<=0&&ccw(p3,p4,p1)*ccw(p3,p4,p2)<=0);
}

bool intersect(Segment s1,Segment s2){
    return intersect(s1.p1,s1.p2,s2.p1,s2.p2);
}

bool overlap(Segment s1,Segment s2){
    int a=ccw(s1.p1,s1.p2,s2.p1),b=ccw(s1.p1,s1.p2,s2.p2);
    if(a&1||b&1) return 0;
    if(a==2){
        if(b==-2||(b==0&&!(s2.p2==s1.p1))) return 1;
        else return 0;
    }
    if(a==-2){
        if(b==2||(b==0&&!(s2.p2==s1.p2))) return 1;
        else return 0;
    }
    if(a==0){
        if(s1.p1==s2.p1){
            if(b!=2) return 1;
            else return 0;
        }
        else if(s1.p2==s2.p1){
            if(b!=-2) return 1;
            else return 0;
        }
        else return 1;
    }
    return 0;
}
//s1とs2の共通の線分(長さ0より大きい)があるかどうか

typedef Segment Line;

double getDistance(Point a,Point b){
    return abs(a-b);
}

double getDistanceLP(Line l,Point p){
    return abs(cross(l.p2-l.p1,p-l.p1)/abs(l.p2-l.p1));
}

double getDistanceSP(Segment s,Point p){
    if(dot(s.p2-s.p1,p-s.p1)<0.0) return abs(p-s.p1);
    if(dot(s.p1-s.p2,p-s.p2)<0.0) return abs(p-s.p2);
    return getDistanceLP(s,p);
}

double getDistance(Segment s1,Segment s2){
    if(intersect(s1,s2)) return 0.0;
    return min({getDistanceSP(s1,s2.p1),getDistanceSP(s1,s2.p2),getDistanceSP(s2,s1.p1),getDistanceSP(s2,s1.p2)});
}

Point getCrossPointS(Segment s1,Segment s2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    Vector base=s2.p2-s2.p1;
    double d1=abs(cross(base,s1.p1-s2.p1));
    double d2=abs(cross(base,s1.p2-s2.p1));
    double t=d1/(d1+d2);
    return s1.p1+(s1.p2-s1.p1)*t;
}//同じ時壊れます

Point getCrossPointL(Line l1,Line l2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    
    Vector v1=l1.p2-l1.p1,v2=l2.p2-l2.p1;
    
    return l1.p1+v1*cross(v2,l2.p1-l1.p1)/cross(v2,v1);
}

Segment ParallelSegment(Segment s,double d){
    Vector v={-(s.p2-s.p1).y,(s.p2-s.p1).x};
    v=v/abs(v);
    
    s.p1=s.p1+v*d;
    s.p2=s.p2+v*d;
    
    return s;
}

Point naisin(Point p1,Point p2,Point p3){
    if(p1==p2&&p2==p3&&p3==p1) return p1;
    
    return (p1*abs(p2-p3)+p2*abs(p1-p3)+p3*abs(p1-p2))/(abs(p2-p3)+abs(p1-p3)+abs(p1-p2));
}

Point naisin(Line l1,Line l2,Line l3){
    //平行でない前提
    
    Point p1=getCrossPointL(l1,l2),p2=getCrossPointL(l1,l3),p3=getCrossPointL(l2,l3);
    return naisin(p1,p2,p3);
}

//ネットの適当を書いたのであってるか全く知りません→あってそう

class Circle{
public:
    Point c;
    double r;
    Circle(Point c=Point(),double r=0.0):c(c),r(r){}
};

Point CircleCenter(Point a,Point b,Point c){
    Point u=a-b,v=a-c;
    double m1=(norm(a)-norm(b))/2.0,m2=(norm(a)-norm(c))/2.0;
    
    Point res;
    if(cross(u,v)==0.0){
        res.x=1e9;
        res.y=1e9;
        
        return res;
    }
    res.x=(m1*v.y-m2*u.y)/cross(u,v);
    res.y=(m1*v.x-m2*u.x)/cross(v,u);
    
    return res;
}
//3点を通る円の中心を返す

//交わる 0
// c1がc2のinside 1
// c1がc2のoutside 2
// 交わらない 3

int not_intersect(Circle c1,Circle c2){
    double d=getDistance(c1.c,c2.c);
    double r1=c1.r,r2=c2.r;
    if(r1<r2){
        if(d<(r2-r1)) return 1;
    }
    if(r1>r2){
        if(d<(r1-r2)) return 2;
    }
    if(d<=r1+r2) return 0;
    else return 3;
}

pair<Point,Point> segCrossPpoints(Circle c,Line l){
    //assert(intersect(c,l));
    Vector pr=project(l,c.c);
    Vector e=(l.p2-l.p1)/abs(l.p2-l.p1);
    double base=sqrt(c.r*c.r-norm(pr-c.c));
    return make_pair(pr+e*base,pr-e*base);
}

double arg(Vector p){return atan2(p.y,p.x);}
Vector polar(double a,double r){return Point(cos(r)*a,sin(r)*a);}

//inside(outside)

pair<Point,Point> getCrossPoints(Circle c1,Circle c2){
    //assert(intersect(c1,c2));
    double d=abs(c1.c-c2.c);
    double a=acos((c1.r*c1.r+d*d-c2.r*c2.r)/(2*c1.r*d));
    double t=arg(c2.c-c1.c);
    return make_pair(c1.c+polar(c1.r,t+a),c1.c+polar(c1.r,t-a));
}

vector<Line> Commontangent(Circle c1,Circle c2){
    vector<Line> res;
    Point p=c2.c-c1.c;
    
    if(abs(p)>=(c1.r+c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r+c2.r)+p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r+c2.r)-p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r+c2.r)-p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r+c2.r)+p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    if(abs(p)>=abs(c1.r-c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r-c2.r)+p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r-c2.r)-p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r-c2.r)-p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r-c2.r)+p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    return res;
}

typedef vector<Point> Polygon;

/*
 IN 2
 ON 1
 OUT 0
 */

int contains(Polygon g,Point p){
    int n=int(g.size());
    bool x=false;
    for(int i=0;i<n;i++){
        Point a=g[i]-p,b=g[(i+1)%n]-p;
        if(a.y>b.y) swap(a,b);
        if(a.y<eps&&0<b.y&&cross(a,b)<0) x=!x;
        if(abs(cross(a,b))<eps&&dot(a,b)<eps) return 1;
    }
    return (x?2:0);
}

Polygon andrewScan(Polygon s,bool ok){
    Polygon u,l;
    sort(all(s));
    
    if(int(s.size())<3) return s;
    int n=int(s.size());
    
    u.push_back(s[0]);
    u.push_back(s[1]);
    
    l.push_back(s[n-1]);
    l.push_back(s[n-2]);
    
    if(ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])==counter_clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])==counter_clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    if(!ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])!=clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])!=clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    reverse(all(l));
    
    for(int i=int(u.size())-2;i>=1;i--) l.push_back(u[i]);
    
    return l;
}//ok==1なら辺の上も含める

Polygon convex_cut(const Polygon& P, const Line& l) {
    Polygon Q;
    for(int i=0;i<si(P);i++){
        Point A=P[i],B=P[(i+1)%si(P)];
        if(ccw(l.p1,l.p2,A)!=-1)Q.push_back(A);
        if(ccw(l.p1,l.p2,A)*ccw(l.p1,l.p2,B)<0) Q.push_back(getCrossPointL(Line{A,B},l));
    }
    return Q;
}

double area(Point a,Point b,Point c){
    b=b-a;
    c=c-a;
    return abs(b.x*c.y-b.y*c.x)/2.0;
}

double area(Polygon &P){
    if(si(P)==0) return 0.0;
    double res=0;
    Point c={0.0,0.0};
    for(int i=0;i<si(P);i++){
        c=c+P[i];
    }
    c=c/si(P);
    
    for(int i=0;i<si(P);i++){
        res+=area(c,P[i],P[(i+1)%si(P)]);
    }
    
    return res;
}

ll gcd(ll a,ll b){
    if(b==0) return a;
    return gcd(b,a%b);
}

pair<Point,Vector> perpendicular_bisector(Point a,Point b){
    Point c=(a+b)/2;
    Vector v=b-c;
    swap(v.x,v.y);
    v.x*=-1;
    
    Point p=c;
    if(v.x==0){
        v.y=1;
        p.y=0;
    }
    else if(v.y==0){
        v.x=1;
        p.x=0;
    }
    else{
        if(v.x<0){
            v.x*=-1;
            v.y*=-1;
        }
        ll g=gcd(abs(ll(v.x)),abs(ll(v.y)));
        v.x/=g;
        v.y/=g;
        if(p.x>=0){
            ll d=p.x/v.x;
            p=p-v*d;
        }else{
            ll d=abs(p.x)/v.x;
            p=p+v*d;
            
            if(p.x<0){
                p=p+v;
            }
        }
    }
    
    return mp(p,v);
}
//2倍するなりして整数にしておくこと

int main() {
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int N;cin>>N;
    vector<Point> P(N);
    for(int i=0;i<N;i++) cin>>P[i].x>>P[i].y;
    bool f=false;
    for(int q=0;q<2;q++){
        bool ok=true;
        for(int i=2;i+1<N;i++){
            Polygon Q;
            for(int j=0;j<=i;j++) Q.push_back(P[j]);
            Q=andrewScan(Q,0);
            Point PP=P[i]+(P[i+1]-P[i])/abs(P[i+1]-P[i])/1e6;
            if(contains(Q,PP)==2) ok=false;
        }
        for(int i=0;i+1<N-2;i++){
            Polygon Q;
            for(int j=i+1;j<N;j++) Q.push_back(P[j]);
            Q=andrewScan(Q,0);
            Point PP=P[i+1]+(P[i]-P[i+1])/abs(P[i]-P[i+1])/1e6;
            if(contains(Q,PP)==2) ok=false;
        }
        if(ok) f=true;
        
        reverse(all(P));
    }
    if(f) cout<<"Possible\n";
    else cout<<"Impossible\n";
}
accuracy: 0.022058823529411766
time_ms: 110
memory_kb: 3560
final_rank: 11

code_snippet: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=305,INF=1<<29;

//幾何ライブラリ
// define double ll をするときは Point の < と == も書き換えよう！

const double eps=1e-8;
const double pi=acos((double)-1.0L);
#define equals(a,b) (fabs((a)-(b))<eps)

double torad(double deg) {return (double)(deg)*pi/180.0;}
double todeg(double ang) {return ang*180.0/pi;}

class Point{
public:
    double x,y;
    
    Point(double x=0,double y=0):x(x),y(y){}
    
    Point operator + (Point p){return Point(x+p.x,y+p.y);}
    Point operator - (Point p){return Point(x-p.x,y-p.y);}
    Point operator * (double a){return Point(a*x,a*y);}
    Point operator / (double a){return Point(x/a,y/a);}
    
    double abs(){return sqrt(norm());}
    double norm(){return x*x+y*y;}
    
    bool operator < (const Point &p)const{
        return x+eps<p.x||(equals(x,p.x)&&y+eps<p.y);
    }
    
    bool operator == (const Point &p)const{
        return fabs(x-p.x)<eps/100000&&fabs(y-p.y)<eps/100000;
    }
};

typedef Point Vector;

double norm(Vector a){
    return a.x*a.x+a.y*a.y;
}

double abs(Vector a){
    return sqrt(norm(a));
}

double dot(Vector a,Vector b){
    return a.x*b.x+a.y*b.y;
}

double cross(Vector a,Vector b){
    return a.x*b.y-a.y*b.x;
}

struct Segment{
    Point p1,p2;
};

bool isOrthogonal(Vector a,Vector b){
    return equals(dot(a,b),0.0);
}

bool isOrthogonal(Point a1,Point a2,Point b1,Point b2){
    return isOrthogonal(a1-a2,b1-b2);
}

bool isOrthogonal(Segment s1,Segment s2){
    return equals(dot(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

bool isParallel(Vector a,Vector b){
    return equals(cross(a,b),0.0);
}

bool isParallel(Point a1,Point a2,Point b1,Point b2){
    return isParallel(a1-a2,b1-b2);
}

bool isParallel(Segment s1,Segment s2){
    return equals(cross(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

Point project(Segment s,Point p){
    Vector base=s.p2-s.p1;
    double r=dot(p-s.p1,base)/norm(base);
    return s.p1+base*r;
}

Point reflect(Segment s,Point p){
    return p+(project(s,p)-p)*2.0;
}

Point turn(Point p,Point c,double pi){
    double q=atan2(p.y-c.y,p.x-c.x);
    q+=pi;
    p=c+Point{cos(q)*abs(p-c),sin(q)*abs(p-c)};
    
    return p;
}
//pをcを中心としてpi回転させる(1周で2π)
//p=cのときnan

//p0,p1,p2の順に見たときどうなるか？

static const int counter_clockwise=1;
static const int clockwise=-1;
static const int online_back=2;
static const int online_front=-2;
static const int on_segment=0;

int ccw(Point p0,Point p1,Point p2){
    Vector a=p1-p0;
    Vector b=p2-p0;
    
    if(cross(a,b)>eps) return counter_clockwise;
    if(cross(a,b)<-eps) return clockwise;
    if(dot(a,b)<-eps) return online_back;
    if(a.norm()<b.norm()) return online_front;
    
    return on_segment;
}

bool intersect(Point p1,Point p2,Point p3,Point p4){
    return(ccw(p1,p2,p3)*ccw(p1,p2,p4)<=0&&ccw(p3,p4,p1)*ccw(p3,p4,p2)<=0);
}

bool intersect(Segment s1,Segment s2){
    return intersect(s1.p1,s1.p2,s2.p1,s2.p2);
}

bool overlap(Segment s1,Segment s2){
    int a=ccw(s1.p1,s1.p2,s2.p1),b=ccw(s1.p1,s1.p2,s2.p2);
    if(a&1||b&1) return 0;
    if(a==2){
        if(b==-2||(b==0&&!(s2.p2==s1.p1))) return 1;
        else return 0;
    }
    if(a==-2){
        if(b==2||(b==0&&!(s2.p2==s1.p2))) return 1;
        else return 0;
    }
    if(a==0){
        if(s1.p1==s2.p1){
            if(b!=2) return 1;
            else return 0;
        }
        else if(s1.p2==s2.p1){
            if(b!=-2) return 1;
            else return 0;
        }
        else return 1;
    }
    return 0;
}
//s1とs2の共通の線分(長さ0より大きい)があるかどうか

typedef Segment Line;

double getDistance(Point a,Point b){
    return abs(a-b);
}

double getDistanceLP(Line l,Point p){
    return abs(cross(l.p2-l.p1,p-l.p1)/abs(l.p2-l.p1));
}

double getDistanceSP(Segment s,Point p){
    if(dot(s.p2-s.p1,p-s.p1)<0.0) return abs(p-s.p1);
    if(dot(s.p1-s.p2,p-s.p2)<0.0) return abs(p-s.p2);
    return getDistanceLP(s,p);
}

double getDistance(Segment s1,Segment s2){
    if(intersect(s1,s2)) return 0.0;
    return min({getDistanceSP(s1,s2.p1),getDistanceSP(s1,s2.p2),getDistanceSP(s2,s1.p1),getDistanceSP(s2,s1.p2)});
}

Point getCrossPointS(Segment s1,Segment s2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    Vector base=s2.p2-s2.p1;
    double d1=abs(cross(base,s1.p1-s2.p1));
    double d2=abs(cross(base,s1.p2-s2.p1));
    double t=d1/(d1+d2);
    return s1.p1+(s1.p2-s1.p1)*t;
}//同じ時壊れます

Point getCrossPointL(Line l1,Line l2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    
    Vector v1=l1.p2-l1.p1,v2=l2.p2-l2.p1;
    
    return l1.p1+v1*cross(v2,l2.p1-l1.p1)/cross(v2,v1);
}

Segment ParallelSegment(Segment s,double d){
    Vector v={-(s.p2-s.p1).y,(s.p2-s.p1).x};
    v=v/abs(v);
    
    s.p1=s.p1+v*d;
    s.p2=s.p2+v*d;
    
    return s;
}

Point naisin(Point p1,Point p2,Point p3){
    if(p1==p2&&p2==p3&&p3==p1) return p1;
    
    return (p1*abs(p2-p3)+p2*abs(p1-p3)+p3*abs(p1-p2))/(abs(p2-p3)+abs(p1-p3)+abs(p1-p2));
}

Point naisin(Line l1,Line l2,Line l3){
    //平行でない前提
    
    Point p1=getCrossPointL(l1,l2),p2=getCrossPointL(l1,l3),p3=getCrossPointL(l2,l3);
    return naisin(p1,p2,p3);
}

//ネットの適当を書いたのであってるか全く知りません→あってそう

class Circle{
public:
    Point c;
    double r;
    Circle(Point c=Point(),double r=0.0):c(c),r(r){}
};

Point CircleCenter(Point a,Point b,Point c){
    Point u=a-b,v=a-c;
    double m1=(norm(a)-norm(b))/2.0,m2=(norm(a)-norm(c))/2.0;
    
    Point res;
    if(cross(u,v)==0.0){
        res.x=1e9;
        res.y=1e9;
        
        return res;
    }
    res.x=(m1*v.y-m2*u.y)/cross(u,v);
    res.y=(m1*v.x-m2*u.x)/cross(v,u);
    
    return res;
}
//3点を通る円の中心を返す

//交わる 0
// c1がc2のinside 1
// c1がc2のoutside 2
// 交わらない 3

int not_intersect(Circle c1,Circle c2){
    double d=getDistance(c1.c,c2.c);
    double r1=c1.r,r2=c2.r;
    if(r1<r2){
        if(d<(r2-r1)) return 1;
    }
    if(r1>r2){
        if(d<(r1-r2)) return 2;
    }
    if(d<=r1+r2) return 0;
    else return 3;
}

pair<Point,Point> segCrossPpoints(Circle c,Line l){
    //assert(intersect(c,l));
    Vector pr=project(l,c.c);
    Vector e=(l.p2-l.p1)/abs(l.p2-l.p1);
    double base=sqrt(c.r*c.r-norm(pr-c.c));
    return make_pair(pr+e*base,pr-e*base);
}

double arg(Vector p){return atan2(p.y,p.x);}
Vector polar(double a,double r){return Point(cos(r)*a,sin(r)*a);}

//inside(outside)

pair<Point,Point> getCrossPoints(Circle c1,Circle c2){
    //assert(intersect(c1,c2));
    double d=abs(c1.c-c2.c);
    double a=acos((c1.r*c1.r+d*d-c2.r*c2.r)/(2*c1.r*d));
    double t=arg(c2.c-c1.c);
    return make_pair(c1.c+polar(c1.r,t+a),c1.c+polar(c1.r,t-a));
}

vector<Line> Commontangent(Circle c1,Circle c2){
    vector<Line> res;
    Point p=c2.c-c1.c;
    
    if(abs(p)>=(c1.r+c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r+c2.r)+p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r+c2.r)-p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r+c2.r)-p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r+c2.r)+p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    if(abs(p)>=abs(c1.r-c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r-c2.r)+p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r-c2.r)-p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r-c2.r)-p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r-c2.r)+p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    return res;
}

typedef vector<Point> Polygon;

/*
 IN 2
 ON 1
 OUT 0
 */

int contains(Polygon g,Point p){
    int n=int(g.size());
    bool x=false;
    for(int i=0;i<n;i++){
        Point a=g[i]-p,b=g[(i+1)%n]-p;
        if(a.y>b.y) swap(a,b);
        if(a.y<eps&&0<b.y&&cross(a,b)<0) x=!x;
        if(abs(cross(a,b))<eps&&dot(a,b)<eps) return 1;
    }
    return (x?2:0);
}

Polygon andrewScan(Polygon s,bool ok){
    Polygon u,l;
    sort(all(s));
    
    if(int(s.size())<3) return s;
    int n=int(s.size());
    
    u.push_back(s[0]);
    u.push_back(s[1]);
    
    l.push_back(s[n-1]);
    l.push_back(s[n-2]);
    
    if(ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])==counter_clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])==counter_clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    if(!ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])!=clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])!=clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    reverse(all(l));
    
    for(int i=int(u.size())-2;i>=1;i--) l.push_back(u[i]);
    
    return l;
}//ok==1なら辺の上も含める

Polygon convex_cut(const Polygon& P, const Line& l) {
    Polygon Q;
    for(int i=0;i<si(P);i++){
        Point A=P[i],B=P[(i+1)%si(P)];
        if(ccw(l.p1,l.p2,A)!=-1)Q.push_back(A);
        if(ccw(l.p1,l.p2,A)*ccw(l.p1,l.p2,B)<0) Q.push_back(getCrossPointL(Line{A,B},l));
    }
    return Q;
}

double area(Point a,Point b,Point c){
    b=b-a;
    c=c-a;
    return abs(b.x*c.y-b.y*c.x)/2.0;
}

double area(Polygon &P){
    if(si(P)==0) return 0.0;
    double res=0;
    Point c={0.0,0.0};
    for(int i=0;i<si(P);i++){
        c=c+P[i];
    }
    c=c/si(P);
    
    for(int i=0;i<si(P);i++){
        res+=area(c,P[i],P[(i+1)%si(P)]);
    }
    
    return res;
}

ll gcd(ll a,ll b){
    if(b==0) return a;
    return gcd(b,a%b);
}

pair<Point,Vector> perpendicular_bisector(Point a,Point b){
    Point c=(a+b)/2;
    Vector v=b-c;
    swap(v.x,v.y);
    v.x*=-1;
    
    Point p=c;
    if(v.x==0){
        v.y=1;
        p.y=0;
    }
    else if(v.y==0){
        v.x=1;
        p.x=0;
    }
    else{
        if(v.x<0){
            v.x*=-1;
            v.y*=-1;
        }
        ll g=gcd(abs(ll(v.x)),abs(ll(v.y)));
        v.x/=g;
        v.y/=g;
        if(p.x>=0){
            ll d=p.x/v.x;
            p=p-v*d;
        }else{
            ll d=abs(p.x)/v.x;
            p=p+v*d;
            
            if(p.x<0){
                p=p+v;
            }
        }
    }
    
    return mp(p,v);
}
//2倍するなりして整数にしておくこと

int main() {
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int N;cin>>N;
    vector<Point> P(N);
    for(int i=0;i<N;i++) cin>>P[i].x>>P[i].y;
    bool f=false;
    for(int q=0;q<2;q++){
        vector<Point> Q={P[0],P[1]};
        bool ok=true;
        for(int i=2;i+1<N;i++){
            Q.push_back(P[i]);
            Q=andrewScan(Q,0);
            Point PP=P[i]+(P[i+1]-P[i])/abs(P[i+1]-P[i])/1e6;
            if(contains(Q,PP)) ok=false;
        }
        for(int i=0;i+1<N-2;i++){
            Polygon R;
            for(int j=i+1;j<N;j++) R.push_back(P[j]);
            R=andrewScan(R,0);
            Point PP=P[i+1]+(P[i]-P[i+1])/abs(P[i]-P[i+1])/1e6;
            if(contains(R,PP)) ok=false;
        }
        if(ok) f=true;
        
        reverse(all(P));
    }
    if(f) cout<<"Possible\n";
    else cout<<"Impossible\n";
}
accuracy: 0.022058823529411766
time_ms: 70
memory_kb: 3648
final_rank: 12

code_snippet: #include <bits/stdc++.h>
using namespace std;
typedef long long ll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return true; } return false; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return true; } return false; }
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define mp make_pair
#define si(x) int(x.size())
const int mod=998244353,MAX=305,INF=1<<29;

//幾何ライブラリ
// define double ll をするときは Point の < と == も書き換えよう！

const double eps=1e-8;
const double pi=acos((double)-1.0L);
#define equals(a,b) (fabs((a)-(b))<eps)

double torad(double deg) {return (double)(deg)*pi/180.0;}
double todeg(double ang) {return ang*180.0/pi;}

class Point{
public:
    double x,y;
    
    Point(double x=0,double y=0):x(x),y(y){}
    
    Point operator + (Point p){return Point(x+p.x,y+p.y);}
    Point operator - (Point p){return Point(x-p.x,y-p.y);}
    Point operator * (double a){return Point(a*x,a*y);}
    Point operator / (double a){return Point(x/a,y/a);}
    
    double abs(){return sqrt(norm());}
    double norm(){return x*x+y*y;}
    
    bool operator < (const Point &p)const{
        return x+eps<p.x||(equals(x,p.x)&&y+eps<p.y);
    }
    
    bool operator == (const Point &p)const{
        return fabs(x-p.x)<eps/100000&&fabs(y-p.y)<eps/100000;
    }
};

typedef Point Vector;

double norm(Vector a){
    return a.x*a.x+a.y*a.y;
}

double abs(Vector a){
    return sqrt(norm(a));
}

double dot(Vector a,Vector b){
    return a.x*b.x+a.y*b.y;
}

double cross(Vector a,Vector b){
    return a.x*b.y-a.y*b.x;
}

struct Segment{
    Point p1,p2;
};

bool isOrthogonal(Vector a,Vector b){
    return equals(dot(a,b),0.0);
}

bool isOrthogonal(Point a1,Point a2,Point b1,Point b2){
    return isOrthogonal(a1-a2,b1-b2);
}

bool isOrthogonal(Segment s1,Segment s2){
    return equals(dot(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

bool isParallel(Vector a,Vector b){
    return equals(cross(a,b),0.0);
}

bool isParallel(Point a1,Point a2,Point b1,Point b2){
    return isParallel(a1-a2,b1-b2);
}

bool isParallel(Segment s1,Segment s2){
    return equals(cross(s1.p2-s1.p1,s2.p2-s2.p1),0.0);
}

Point project(Segment s,Point p){
    Vector base=s.p2-s.p1;
    double r=dot(p-s.p1,base)/norm(base);
    return s.p1+base*r;
}

Point reflect(Segment s,Point p){
    return p+(project(s,p)-p)*2.0;
}

Point turn(Point p,Point c,double pi){
    double q=atan2(p.y-c.y,p.x-c.x);
    q+=pi;
    p=c+Point{cos(q)*abs(p-c),sin(q)*abs(p-c)};
    
    return p;
}
//pをcを中心としてpi回転させる(1周で2π)
//p=cのときnan

//p0,p1,p2の順に見たときどうなるか？

static const int counter_clockwise=1;
static const int clockwise=-1;
static const int online_back=2;
static const int online_front=-2;
static const int on_segment=0;

int ccw(Point p0,Point p1,Point p2){
    Vector a=p1-p0;
    Vector b=p2-p0;
    
    if(cross(a,b)>eps) return counter_clockwise;
    if(cross(a,b)<-eps) return clockwise;
    if(dot(a,b)<-eps) return online_back;
    if(a.norm()<b.norm()) return online_front;
    
    return on_segment;
}

bool intersect(Point p1,Point p2,Point p3,Point p4){
    return(ccw(p1,p2,p3)*ccw(p1,p2,p4)<=0&&ccw(p3,p4,p1)*ccw(p3,p4,p2)<=0);
}

bool intersect(Segment s1,Segment s2){
    return intersect(s1.p1,s1.p2,s2.p1,s2.p2);
}

bool overlap(Segment s1,Segment s2){
    int a=ccw(s1.p1,s1.p2,s2.p1),b=ccw(s1.p1,s1.p2,s2.p2);
    if(a&1||b&1) return 0;
    if(a==2){
        if(b==-2||(b==0&&!(s2.p2==s1.p1))) return 1;
        else return 0;
    }
    if(a==-2){
        if(b==2||(b==0&&!(s2.p2==s1.p2))) return 1;
        else return 0;
    }
    if(a==0){
        if(s1.p1==s2.p1){
            if(b!=2) return 1;
            else return 0;
        }
        else if(s1.p2==s2.p1){
            if(b!=-2) return 1;
            else return 0;
        }
        else return 1;
    }
    return 0;
}
//s1とs2の共通の線分(長さ0より大きい)があるかどうか

typedef Segment Line;

double getDistance(Point a,Point b){
    return abs(a-b);
}

double getDistanceLP(Line l,Point p){
    return abs(cross(l.p2-l.p1,p-l.p1)/abs(l.p2-l.p1));
}

double getDistanceSP(Segment s,Point p){
    if(dot(s.p2-s.p1,p-s.p1)<0.0) return abs(p-s.p1);
    if(dot(s.p1-s.p2,p-s.p2)<0.0) return abs(p-s.p2);
    return getDistanceLP(s,p);
}

double getDistance(Segment s1,Segment s2){
    if(intersect(s1,s2)) return 0.0;
    return min({getDistanceSP(s1,s2.p1),getDistanceSP(s1,s2.p2),getDistanceSP(s2,s1.p1),getDistanceSP(s2,s1.p2)});
}

Point getCrossPointS(Segment s1,Segment s2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    Vector base=s2.p2-s2.p1;
    double d1=abs(cross(base,s1.p1-s2.p1));
    double d2=abs(cross(base,s1.p2-s2.p1));
    double t=d1/(d1+d2);
    return s1.p1+(s1.p2-s1.p1)*t;
}//同じ時壊れます

Point getCrossPointL(Line l1,Line l2){
    //if(ccw(s1.p1,s1.p2,s2.p1)==0&&ccw(s1.p1,s1.p2,s2.p2)==0) return s1.p1;
    
    Vector v1=l1.p2-l1.p1,v2=l2.p2-l2.p1;
    
    return l1.p1+v1*cross(v2,l2.p1-l1.p1)/cross(v2,v1);
}

Segment ParallelSegment(Segment s,double d){
    Vector v={-(s.p2-s.p1).y,(s.p2-s.p1).x};
    v=v/abs(v);
    
    s.p1=s.p1+v*d;
    s.p2=s.p2+v*d;
    
    return s;
}

Point naisin(Point p1,Point p2,Point p3){
    if(p1==p2&&p2==p3&&p3==p1) return p1;
    
    return (p1*abs(p2-p3)+p2*abs(p1-p3)+p3*abs(p1-p2))/(abs(p2-p3)+abs(p1-p3)+abs(p1-p2));
}

Point naisin(Line l1,Line l2,Line l3){
    //平行でない前提
    
    Point p1=getCrossPointL(l1,l2),p2=getCrossPointL(l1,l3),p3=getCrossPointL(l2,l3);
    return naisin(p1,p2,p3);
}

//ネットの適当を書いたのであってるか全く知りません→あってそう

class Circle{
public:
    Point c;
    double r;
    Circle(Point c=Point(),double r=0.0):c(c),r(r){}
};

Point CircleCenter(Point a,Point b,Point c){
    Point u=a-b,v=a-c;
    double m1=(norm(a)-norm(b))/2.0,m2=(norm(a)-norm(c))/2.0;
    
    Point res;
    if(cross(u,v)==0.0){
        res.x=1e9;
        res.y=1e9;
        
        return res;
    }
    res.x=(m1*v.y-m2*u.y)/cross(u,v);
    res.y=(m1*v.x-m2*u.x)/cross(v,u);
    
    return res;
}
//3点を通る円の中心を返す

//交わる 0
// c1がc2のinside 1
// c1がc2のoutside 2
// 交わらない 3

int not_intersect(Circle c1,Circle c2){
    double d=getDistance(c1.c,c2.c);
    double r1=c1.r,r2=c2.r;
    if(r1<r2){
        if(d<(r2-r1)) return 1;
    }
    if(r1>r2){
        if(d<(r1-r2)) return 2;
    }
    if(d<=r1+r2) return 0;
    else return 3;
}

pair<Point,Point> segCrossPpoints(Circle c,Line l){
    //assert(intersect(c,l));
    Vector pr=project(l,c.c);
    Vector e=(l.p2-l.p1)/abs(l.p2-l.p1);
    double base=sqrt(c.r*c.r-norm(pr-c.c));
    return make_pair(pr+e*base,pr-e*base);
}

double arg(Vector p){return atan2(p.y,p.x);}
Vector polar(double a,double r){return Point(cos(r)*a,sin(r)*a);}

//inside(outside)

pair<Point,Point> getCrossPoints(Circle c1,Circle c2){
    //assert(intersect(c1,c2));
    double d=abs(c1.c-c2.c);
    double a=acos((c1.r*c1.r+d*d-c2.r*c2.r)/(2*c1.r*d));
    double t=arg(c2.c-c1.c);
    return make_pair(c1.c+polar(c1.r,t+a),c1.c+polar(c1.r,t-a));
}

vector<Line> Commontangent(Circle c1,Circle c2){
    vector<Line> res;
    Point p=c2.c-c1.c;
    
    if(abs(p)>=(c1.r+c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r+c2.r)+p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r+c2.r)-p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r+c2.r)-p.y*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r+c2.r)+p.x*sqrt(norm(p)-(c1.r+c2.r)*(c1.r+c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    if(abs(p)>=abs(c1.r-c2.r)){
        Point a,b;
        a.x=c1.r*(p.x*(c1.r-c2.r)+p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        a.y=c1.r*(p.y*(c1.r-c2.r)-p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        b.x=c1.r*(p.x*(c1.r-c2.r)-p.y*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        b.y=c1.r*(p.y*(c1.r-c2.r)+p.x*sqrt(norm(p)-(c1.r-c2.r)*(c1.r-c2.r)))/norm(p);
        
        res.push_back(Line{a+c1.c,a+c1.c+Point{-a.y,a.x}});
        if(!(a==b)){
            res.push_back(Line{b+c1.c,b+c1.c+Point{-b.y,b.x}});
        }
    }
    
    return res;
}

typedef vector<Point> Polygon;

/*
 IN 2
 ON 1
 OUT 0
 */

int contains(Polygon g,Point p){
    int n=int(g.size());
    bool x=false;
    for(int i=0;i<n;i++){
        Point a=g[i]-p,b=g[(i+1)%n]-p;
        if(a.y>b.y) swap(a,b);
        if(a.y<eps&&0<b.y&&cross(a,b)<0) x=!x;
        if(abs(cross(a,b))<eps&&dot(a,b)<eps) return 1;
    }
    return (x?2:0);
}

Polygon andrewScan(Polygon s,bool ok){
    Polygon u,l;
    sort(all(s));
    
    if(int(s.size())<3) return s;
    int n=int(s.size());
    
    u.push_back(s[0]);
    u.push_back(s[1]);
    
    l.push_back(s[n-1]);
    l.push_back(s[n-2]);
    
    if(ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])==counter_clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])==counter_clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    if(!ok){
        for(int i=2;i<n;i++){
            for(int j=int(u.size());j>=2&&ccw(u[j-2],u[j-1],s[i])!=clockwise;j--){
                u.pop_back();
            }
            u.push_back(s[i]);
        }
        
        for(int i=int(s.size())-3;i>=0;i--){
            for(int j=int(l.size());j>=2&&ccw(l[j-2],l[j-1],s[i])!=clockwise;j--){
                l.pop_back();
            }
            l.push_back(s[i]);
        }
    }
    
    reverse(all(l));
    
    for(int i=int(u.size())-2;i>=1;i--) l.push_back(u[i]);
    
    return l;
}//ok==1なら辺の上も含める

Polygon convex_cut(const Polygon& P, const Line& l) {
    Polygon Q;
    for(int i=0;i<si(P);i++){
        Point A=P[i],B=P[(i+1)%si(P)];
        if(ccw(l.p1,l.p2,A)!=-1)Q.push_back(A);
        if(ccw(l.p1,l.p2,A)*ccw(l.p1,l.p2,B)<0) Q.push_back(getCrossPointL(Line{A,B},l));
    }
    return Q;
}

double area(Point a,Point b,Point c){
    b=b-a;
    c=c-a;
    return abs(b.x*c.y-b.y*c.x)/2.0;
}

double area(Polygon &P){
    if(si(P)==0) return 0.0;
    double res=0;
    Point c={0.0,0.0};
    for(int i=0;i<si(P);i++){
        c=c+P[i];
    }
    c=c/si(P);
    
    for(int i=0;i<si(P);i++){
        res+=area(c,P[i],P[(i+1)%si(P)]);
    }
    
    return res;
}

ll gcd(ll a,ll b){
    if(b==0) return a;
    return gcd(b,a%b);
}

pair<Point,Vector> perpendicular_bisector(Point a,Point b){
    Point c=(a+b)/2;
    Vector v=b-c;
    swap(v.x,v.y);
    v.x*=-1;
    
    Point p=c;
    if(v.x==0){
        v.y=1;
        p.y=0;
    }
    else if(v.y==0){
        v.x=1;
        p.x=0;
    }
    else{
        if(v.x<0){
            v.x*=-1;
            v.y*=-1;
        }
        ll g=gcd(abs(ll(v.x)),abs(ll(v.y)));
        v.x/=g;
        v.y/=g;
        if(p.x>=0){
            ll d=p.x/v.x;
            p=p-v*d;
        }else{
            ll d=abs(p.x)/v.x;
            p=p+v*d;
            
            if(p.x<0){
                p=p+v;
            }
        }
    }
    
    return mp(p,v);
}
//2倍するなりして整数にしておくこと

int main() {
    
    std::ifstream in("text.txt");
    std::cin.rdbuf(in.rdbuf());
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int N;cin>>N;
    vector<Point> P(N);
    for(int i=0;i<N;i++) cin>>P[i].x>>P[i].y;
    bool f=false;
    for(int q=0;q<2;q++){
        vector<Point> Q={P[0],P[1]};
        bool ok=true;
        for(int i=2;i+1<N;i++){
            Q.push_back(P[i]);
            Q=andrewScan(Q,0);
            Point PP=P[i]+(P[i+1]-P[i])/abs(P[i+1]-P[i])/1e6;
            if(contains(Q,PP)) ok=false;
        }
        if(ok) f=true;
        
        reverse(all(P));
    }
    if(f) cout<<"Possible\n";
    else cout<<"Impossible\n";
}
accuracy: 0.007352941176470588
time_ms: 30
memory_kb: 3584
final_rank: 17

code_snippet: #include <bits/stdc++.h>
using namespace std;

#define rep(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define ALL(v) (v).begin(),(v).end()
using ll=long long int;
const int inf = 0x3fffffff; const ll INF = 0x1fffffffffffffff; const double eps=1e-12;
template<typename T>inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template<typename T>inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}

using T=double;
using P=complex<T>;
using Poly=vector<P>;
#define X real()
#define Y imag()
inline bool eq(const T& a,const T& b){
   return fabs(a-b)<eps;
}
inline bool cmp(const P& a,const P& b){
   return (eq(a.X,b.X)?a.Y<b.Y:a.X<b.X);
}
struct Line{
   P a,b;
   Line(){}
   Line(P _a,P _b):a(_a),b(_b){}
   Line(T A,T B,T C){
      if(eq(A,.0)){
         a=P(0,C/B),b=P(1/C/B);
      }
      else if(eq(B,.0)){
         a=P(C/A,0),b=P(C/A,1);
      }
      else{
         a=P(0,C/B),b=P(C/A,0);
      }
   }
};
struct Segment:Line{
   Segment(){}
   Segment(P _a,P _b):Line(_a,_b){}
};
struct Circle{
   P p; T r;
   Circle(){}
   Circle(P _p,T _r):p(_p),r(_r){}
};

istream& operator>>(istream& is,P& p){
   T x,y; is>>x>>y; p=P(x,y);
   return is;
}
ostream& operator<<(ostream& os,P& p){
   os<<fixed<<setprecision(12)<<p.X<<' '<<p.Y;
   return os;
}
P unit(const P& a){return a/abs(a);}
T dot(const P& a,const P& b){
   return a.X*b.X+a.Y*b.Y;
}
T cross(const P& a,const P& b){
   return a.X*b.Y-a.Y*b.X;
}
P rot(const P& a,const T& theta){
   return P(cos(theta)*a.X-sin(theta)*a.Y,
      sin(theta)*a.X+cos(theta)*a.Y);
}
T arg(const P& a,const P& b,const P& c){
   return acos(dot(a-b,c-b)/abs(a-b)/abs(c-b));
}

P Projection(const Line&l,const P& p){
   T t=dot(p-l.a,l.a-l.b)/norm(l.a-l.b);
   return l.a+(l.a-l.b)*t;
}
P Projection(const Segment&l,const P& p){
   T t=dot(p-l.a,l.a-l.b)/norm(l.a-l.b);
   return l.a+(l.a-l.b)*t;
}
P Reflection(const Line& l,const P& p){
   return p+(Projection(l,p)-p)*2.;
}
int ccw(const P& a,P b,P c){
   b-=a; c-=a;
   if(cross(b,c)>eps)return 1; //ccw
   if(cross(b,c)<-eps)return -1; //cw
   if(dot(b,c)<0)return 2; //c,a,b
   if(norm(b)<norm(c))return -2; //a,b,c
   return 0; //a,c,b
}
bool isOrthogonal(const Line& a,const Line& b){
   return eq(dot(a.b-a.a,b.b-b.a),.0);
}
bool isParallel(const Line& a,const Line& b){
   return eq(cross(a.b-a.a,b.b-b.a),.0);
}
bool isIntersect(const Segment& a,const Segment& b){
   return ccw(a.a,a.b,b.a)*ccw(a.a,a.b,b.b)<=0 and
      ccw(b.a,b.b,a.a)*ccw(b.a,b.b,a.b)<=0;
}
int isIntersect(const Circle& a,const Circle& b){
   T d=abs(a.p-b.p);
   if(d>a.r+b.r+eps)return 4;
   if(eq(d,a.r+b.r))return 3;
   if(eq(d,abs(a.r-b.r)))return 1;
   if(d<abs(a.r-b.r)-eps)return 0;
   return 2;
}
T Dist(const Line& a,const P& b){
   P c=Projection(a,b);
   return abs(c-b);
}
T Dist(const Segment& a,const P& b){
   if(dot(a.b-a.a,b-a.a)<eps)return abs(b-a.a);
   if(dot(a.a-a.b,b-a.b)<eps)return abs(b-a.b);
   return abs(cross(a.b-a.a,b-a.a))/abs(a.b-a.a);
}
T Dist(const Segment& a,const Segment& b){
   if(isIntersect(a,b))return .0;
   T res=min({Dist(a,b.a),Dist(a,b.b),Dist(b,a.a),Dist(b,a.b)});
   return res;
}
P Intersection(const Line& a,const Line& b){
   T d1=cross(a.b-a.a,b.b-b.a);
   T d2=cross(a.b-a.a,a.b-b.a);
   if(eq(d1,0) and eq(d2,0))return b.a;
   return b.a+(b.b-b.a)*(d2/d1);
}
Poly Intersection(const Circle& a,const Line& b){
   Poly res;
   T d=Dist(b,a.p);
   if(d>a.r+eps)return res;
   P h=Projection(b,a.p);
   if(eq(d,a.r)){
      res.push_back(h);
      return res;
   }
   P e=unit(b.b-b.a);
   T ph=sqrt(a.r*a.r-d*d);
   res.push_back(h-e*ph);
   res.push_back(h+e*ph);
   return res;
}
Poly Intersection(const Circle& a,const Segment& b){
   Line c(b.a,b.b);
   Poly sub=Intersection(a,c);
   double xmi=min(b.a.X,b.b.X),xma=max(b.a.X,b.b.X);
   double ymi=min(b.a.Y,b.b.Y),yma=max(b.a.Y,b.b.Y);
   Poly res;
   rep(i,0,sub.size()){
      if(xmi<=sub[i].X+eps and sub[i].X-eps<=xma and
         ymi<=sub[i].Y+eps and sub[i].Y-eps<=yma){
         res.push_back(sub[i]);
      }
   }
   return res;
}
Poly Intersection(const Circle& a,const Circle& b){
   Poly res;
   int mode=isIntersect(a,b);
   T d=abs(a.p-b.p);
   if(mode==4 or mode==0)return res;
   if(mode==3){
      T t=a.r/(a.r+b.r);
      res.push_back(a.p+(b.p-a.p)*t);
      return res;
   }
   if(mode==1){
      if(b.r<a.r-eps){
         res.push_back(a.p+(b.p-a.p)*(a.r/d));
      }
      else{
         res.push_back(b.p+(a.p-b.p)*(b.r/d));
      }
      return res;
   }
   T rc=(a.r*a.r+d*d-b.r*b.r)/d/2.;
   T rs=sqrt(a.r*a.r-rc*rc);
   if(a.r-abs(rc)<eps)rs=0;
   P e=unit(b.p-a.p);
   res.push_back(a.p+rc*e+rs*e*P(0,1));
   res.push_back(a.p+rc*e+rs*e*P(0,-1));
   return res;
}
T Area(const Poly& a){
   T res=0;
   int n=a.size();
   rep(i,0,n)res+=cross(a[i],a[(i+1)%n]);
   return fabs(res/2.);
}
T Area(const Poly& a,const Circle& b){
   int n=a.size();
   if(n<3)return .0;
   auto rec=[&](auto self,const Circle& c,const P& p1,const P& p2){
      P va=c.p-p1,vb=c.p-p2;
      T f=cross(va,vb),res=.0;
      if(eq(f,.0))return res;
      if(max(abs(va),abs(vb))<c.r+eps)return f;
      if(Dist(Segment(p1,p2),c.p)>c.r-eps)return c.r*c.r*arg(vb*conj(va));
      auto u=Intersection(c,Segment(p1,p2));
      Poly sub;
      sub.push_back(p1);
      for(auto& x:u)sub.push_back(x);
      sub.push_back(p2);
      rep(i,0,sub.size()-1)res+=self(self,c,sub[i],sub[i+1]);
      return res;
   };
   T res=.0;
   rep(i,0,n)res+=rec(rec,b,a[i],a[(i+1)%n]);
   return fabs(res/2.);
}
T Area(const Circle& a,const Circle& b){
   T d=abs(a.p-b.p);
   if(d>=a.r+b.r-eps)return .0;
   if(d<=abs(a.r-b.r)+eps){
      T r=min(a.r,b.r);
      return M_PI*r*r;
   }
   T ath=acos((a.r*a.r+d*d-b.r*b.r)/d/a.r/2.);
   T res=a.r*a.r*(ath-sin(ath*2)/2.);
   T bth=acos((b.r*b.r+d*d-a.r*a.r)/d/b.r/2.);
   res+=b.r*b.r*(bth-sin(bth*2)/2.);
   return fabs(res);
}
bool isConvex(const Poly& a){
   int n=a.size();
   int cur,pre,nxt;
   rep(i,0,n){
      pre=(i-1+n)%n;
      nxt=(i+1)%n;
      cur=i;
      if(ccw(a[pre],a[cur],a[nxt])==-1)return 0;
   }
   return 1;
}
int isContained(const Poly& a,const P& b){
   bool res=0;
   int n=a.size();
   rep(i,0,n){
      P p=a[i]-b,q=a[(i+1)%n]-b;
      if(p.Y>q.Y)swap(p,q);
      if(p.Y<eps and eps<q.Y and cross(p,q)>eps)res^=1;
      if(eq(cross(p,q),.0) and dot(p,q)<eps)return 1;
   }
   return (res?2:0);
}
Poly ConvexHull(Poly& a){
   int n=a.size(),k=0;
   sort(ALL(a),[](const P& p,const P& q){
      return (eq(p.Y,q.Y)?p.X<q.X:p.Y<q.Y);
   });
   Poly res(n*2);
   for(int i=0;i<n;res[k++]=a[i++]){
      while(k>=2 and cross(res[k-1]-res[k-2],a[i]-res[k-1])<-eps)k--;
   }
   for(int i=n-2,t=k+1;i>=0;res[k++]=a[i--]){
      while(k>=t and cross(res[k-1]-res[k-2],a[i]-res[k-1])<-eps)k--;
   }
   res.resize(k-1); return res;
}
T Diam(const Poly& a){
   int n=a.size();
   int x=0,y=0;
   rep(i,1,n){
      if(a[i].Y>a[x].Y)x=i;
      if(a[i].Y<a[y].Y)y=i;
   }
   T res=abs(a[x]-a[y]);
   int i=x,j=y;
   do{
      if(cross(a[(i+1)%n]-a[i],a[(j+1)%n]-a[j])<0)i=(i+1)%n;
      else j=(j+1)%n;
      chmax(res,abs(a[i]-a[j]));
   }while(i!=x or j!=y);
   return res;
}
Poly Cut(const Poly& a,const Line& l){
   int n=a.size(); Poly res;
   rep(i,0,n){
      P p=a[i],q=a[(i+1)%n];
      if(ccw(l.a,l.b,p)!=-1)res.push_back(p);
      if(ccw(l.a,l.b,p)*ccw(l.a,l.b,q)<0)res.push_back(Intersection(Line(p,q),l));
   }
   return res;
}

T Closest(Poly& a){
   int n=a.size();
   if(n<=1)return 0;
   sort(ALL(a),cmp);
   Poly buf(n);
   auto rec=[&](auto self,int lb,int rb)->T{
      if(rb-lb<=1)return (T)INF;
      int mid=(lb+rb)>>1;
      auto x=a[mid].X;
      T res=min(self(self,lb,mid),self(self,mid,rb));
      inplace_merge(a.begin()+lb,a.begin()+mid,a.begin()+rb,
         [&](auto p,auto q){return p.Y<q.Y;});
      int ptr=0;
      rep(i,lb,rb){
         if(abs(a[i].X-x)>=res)continue;
         rep(j,0,ptr){
            auto sub=a[i]-buf[ptr-1-j];
            if(sub.Y>=res)break;
            chmin(res,abs(sub));
         }
         buf[ptr++]=a[i];
      }
      return res;
   };
   return rec(rec,0,n);
}

Circle Incircle(const P& a,const P& b,const P& c){
   T A=abs(b-c),B=abs(c-a),C=abs(a-b);
   P p(A*a.X+B*b.X+C*c.X,A*a.Y+B*b.Y+C*c.Y);
   p/=(A+B+C);
   T r=Dist(Line(a,b),p);
   return Circle(p,r);
}
Circle Circumcircle(const P& a,const P& b,const P& c){
   Line l1((a+b)/2.,(a+b)/2.+(b-a)*P(0,1));
   Line l2((b+c)/2.,(b+c)/2.+(c-b)*P(0,1));
   P p=Intersection(l1,l2);
   return Circle(p,abs(p-a));
}
Poly tangent(const P& a,const Circle& b){
   return Intersection(b,Circle(a,sqrt(norm(b.p-a)-b.r*b.r)));
}
vector<Line> tangent(const Circle& a,const Circle& b){
   vector<Line> res;
   T d=abs(a.p-b.p);
   if(eq(d,0))return res;
   P u=unit(b.p-a.p);
   P v=u*P(0,1);
   for(int t:{-1,1}){
      T h=(a.r+b.r*t)/d;
      if(eq(h*h,1)){
         res.push_back(Line(a.p+(h>0?u:-u)*a.r,
            a.p+(h>0?u:-u)*a.r+v));
      }
      else if(1>h*h){
         P U=u*h,V=v*sqrt(1-h*h);
         res.push_back(Line(a.p+(U+V)*a.r,b.p-(U+V)*(b.r*t)));
         res.push_back(Line(a.p+(U-V)*a.r,b.p-(U-V)*(b.r*t)));
      }
   }
   return res;
}

int main(){
    int n;
    cin>>n;
    vector<P> p(n);
    rep(i,0,n){
        T x,y;
        cin>>x>>y;
        p[i]=P(x,y);
    }
    
    Poly cur;
    rep(i,0,n){
        if(!cur.empty() and isContained(cur,p[i])){
            puts("Impossible");
            return 0;
        }
        cur.push_back(p[i]);
        if(cur.size()>3)cur=ConvexHull(cur);
    }
    reverse(ALL(p));
    cur.clear();
    rep(i,0,n){
        if(!cur.empty() and isContained(cur,p[i])){
            puts("Impossible");
            return 0;
        }
        cur.push_back(p[i]);
        if(cur.size()>3)cur=ConvexHull(cur);
    }
    puts("Possible");
    return 0;
}
accuracy: 1
time_ms: 30
memory_kb: 3660
final_rank: 4

code_snippet: #include <bits/stdc++.h>

using namespace std;

const string Possible = "Possible";
const string Impossible = "Impossible";

const long double PI = acosl(-1);

int main() {
    int n;
    cin >> n;
    vector<pair<int, int>> ps(n);
    for (auto &[x, y] : ps) cin >> x >> y;

    for (int i = 1; i < n - 1; ++i) {
        auto [cx, cy] = ps[i];

        vector<pair<long double, bool>> args;
        for (int j = 0; j < n; ++j) {
            if (i == j) continue;
            auto [x, y] = ps[j];
            long double t = atan2l(y - cy, x - cx);
            bool f = j > i;
            args.emplace_back(t, f);
            args.emplace_back(t + 2 * PI, f);
        }
        sort(args.begin(), args.end());
        vector<int> sum(2 * (n - 1) + 1, 0);
        for (int p = 0; p < 2 * (n - 1); ++p) {
            sum[p + 1] = sum[p] + args[p].second;
        }
        const int c0 = i, c1 = n - (i + 1);
        bool leq_pi_0 = false, leq_pi_1 = false;
        for (int l = 0, r = 0; l < n - 1; ++l) {
            auto [t, f] = args[l];
            while (args[r].first < t + PI) ++r;
            if (f) {
                int num = sum[r] - sum[l];
                if (num != c1) continue;
                leq_pi_1 = true;
                if (sum[l + num] != sum[r]) {
                    cout << Impossible << endl;
                    return 0;
                }
            } else {
                int num = (r - l) - (sum[r] - sum[l]);
                if (num != c0) continue;
                leq_pi_0  = true;
                if (sum[l + num] != sum[l]) {
                    cout << Impossible << endl;
                    return 0;
                }
            }
        }
        if (not (leq_pi_0 and leq_pi_1)) {
            cout << Impossible << endl;
            return 0;
        }
    }
    cout << Possible << endl;
    return 0;
}
accuracy: 1
time_ms: 150
memory_kb: 3536
final_rank: 1