// {{{ header
#include <bits/stdc++.h>
using namespace std;

using i8   = int8_t;
using u8   = uint8_t;
using i16  = int16_t;
using u16  = uint16_t;
using i32  = int32_t;
using u32  = uint32_t;
using i64  = int64_t;
using u64  = uint64_t;
using i128 = __int128_t;
using u128 = __uint128_t;

using f32  = float;
using f64  = double;
using f80  = __float80;
using f128 = __float128;
// }}}

constexpr i64 INF = 1'010'000'000'000'000'000LL;

constexpr i64 MOD = 1'000'000'007LL;

constexpr f64 EPS = 1e-12;

constexpr f64 PI = 3.14159265358979323846;

// {{{ util
#define FOR(i, start, end) for(i64 i = (start), i##_end=(end); i < i##_end; ++i)
#define REP(i, n) FOR(i, 0, n)

#define ALL(f,c,...) (([&](decltype((c)) cccc) { return (f)(begin(cccc), end(cccc), ## __VA_ARGS__); })(c))
#define SLICE(f,c,l,r,...) (([&](decltype((c)) cccc, decltype((l)) llll, decltype((r)) rrrr) {\
    auto iiii = llll <= rrrr ? begin(cccc)+llll : end(cccc);\
    auto jjjj = llll <= rrrr ? begin(cccc)+rrrr : end(cccc);\
    return (f)(iiii, jjjj, ## __VA_ARGS__);\
})(c,l,r))

#define GENERIC(f) ([](auto&&... args) -> decltype(auto) { return (f)(forward<decltype(args)>(args)...); })

template<typename C>
i64 SIZE(const C& c) { return static_cast<i64>(c.size()); }

template<typename T, size_t N>
i64 SIZE(const T (&)[N]) { return static_cast<i64>(N); }

bool is_odd (i64 x) { return x % 2 != 0; }
bool is_even(i64 x) { return x % 2 == 0; }

template<typename T> i64 cmp(T x, T y) { return (y<x) - (x<y); }
template<typename T> i64 sgn(T x) { return cmp(x, T(0)); }

// Haskell の divMod と同じ
pair<i64,i64> divmod(i64 a, i64 b) {
    i64 q = a / b;
    i64 r = a % b;
    if((b>0 && r<0) || (b<0 && r>0)) {
        --q;
        r += b;
    }
    return {q,r};
}

i64 div_ceil(i64 a, i64 b) {
    i64 q = a / b;
    i64 r = a % b;
    if((b>0 && r>0) || (b<0 && r<0))
        ++q;
    return q;
}

i64 div_floor(i64 a, i64 b) {
    return divmod(a,b).first;
}

i64 modulo(i64 a, i64 b) {
    return divmod(a,b).second;
}

bool feq(f64 x, f64 y, f64 eps=EPS) {
    return fabs(x-y) < eps;
}

template<typename T, typename U>
bool chmax(T& xmax, const U& x) {
    if(xmax < x) {
        xmax = x;
        return true;
    }
    else {
        return false;
    }
}

template<typename T, typename U>
bool chmin(T& xmin, const U& x) {
    if(x < xmin) {
        xmin = x;
        return true;
    }
    else {
        return false;
    }
}

template<typename InputIt>
auto SUM(InputIt first, InputIt last) {
    using T = typename iterator_traits<InputIt>::value_type;
    return accumulate(first, last, T());
}

template<typename ForwardIt, typename UnaryOperation>
ForwardIt transform_self(ForwardIt first, ForwardIt last, UnaryOperation op) {
    return transform(first, last, first, op);
}

template<typename C>
void UNIQ(C& c) {
    c.erase(ALL(unique,c), end(c));
}

template<typename BinaryFunc, typename UnaryFunc>
auto ON(BinaryFunc bf, UnaryFunc uf) {
    return [bf,uf](const auto& x, const auto& y) {
        return bf(uf(x), uf(y));
    };
}

template<typename T>
void RD(T& x) {
    cin >> x;
#ifdef PROCON_LOCAL
    assert(cin);
#endif
}

template<typename T>
void RD(vector<T>& v, i64 n) {
    v.reserve(n);
    REP(_, n) {
        T e; RD(e);
        v.emplace_back(e);
    }
}

template<typename T>
ostream& operator<<(ostream& out, const vector<T>& v) {
    for(auto first = begin(v), it = first; it != end(v); ++it) {
        if(it != first)
            out << ' ';
        out << *it;
    }
    return out;
}

template<typename T1, typename T2>
ostream& operator<<(ostream& out, const pair<T1,T2>& p) {
    return out << '(' << p.first << ',' << p.second << ')';
}

void PRINT() {}

template<typename T, typename... TS>
void PRINT(const T& x, const TS& ...args) {
    cout << x;
    if(sizeof...(args)) {
        cout << ' ';
        PRINT(args...);
    }
}

template<typename... TS>
void PRINTLN(const TS& ...args) {
    PRINT(args...);
    cout << '\n';
}

template<typename T>
void DBG_IMPL(i64 line, const char* expr, const T& value) {
#ifdef PROCON_LOCAL
    cerr << "[L " << line << "]: ";
    cerr << expr << " = " << value << "\n";
#endif
}

#define DBG(expr) DBG_IMPL(__LINE__, #expr, (expr))
// }}}

// {{{ init
struct ProconInit {
    static constexpr int IOS_PREC = 15;
    static constexpr bool AUTOFLUSH = false;

    ProconInit() {
        cin.tie(nullptr);
        ios::sync_with_stdio(false);
        cout << fixed << setprecision(IOS_PREC);
#ifdef PROCON_LOCAL
        cerr << fixed << setprecision(IOS_PREC);
#endif
        if(AUTOFLUSH)
            cout << unitbuf;
    }
} PROCON_INIT;
// }}}

struct Vector {
    f64 x, y;

    Vector() : Vector(0,0) {}
    Vector(f64 xx, f64 yy) : x(xx), y(yy) {}

    const Vector operator-() const {
        return Vector(-x,-y);
    }
    Vector& operator+=(const Vector& rhs) {
        x += rhs.x;
        y += rhs.y;
        return *this;
    }
    Vector& operator-=(const Vector& rhs) {
        x -= rhs.x;
        y -= rhs.y;
        return *this;
    }
    Vector& operator*=(f64 rhs) {
        x *= rhs;
        y *= rhs;
        return *this;
    }
    Vector& operator/=(f64 rhs) {
        x /= rhs;
        y /= rhs;
        return *this;
    }

    f64 norm() const { return x*x + y*y; }
    f64 abs() const { return sqrt(norm()); }

    Vector unit() const { return Vector(*this) /= this->abs(); }
};

const Vector operator+(const Vector& lhs, const Vector& rhs) { return Vector(lhs) += rhs; }
const Vector operator-(const Vector& lhs, const Vector& rhs) { return Vector(lhs) -= rhs; }
const Vector operator*(const Vector& lhs, f64 rhs) { return Vector(lhs) *= rhs; }
const Vector operator*(f64 lhs, const Vector& rhs) { return Vector(rhs) *= lhs; }
const Vector operator/(const Vector& lhs, f64 rhs) { return Vector(lhs) /= rhs; }

bool operator==(const Vector& lhs, const Vector& rhs) {
    return feq(lhs.x,rhs.x) && feq(lhs.y,rhs.y);
}

ostream& operator<<(ostream& out, const Vector& v) {
    return out << "Vector(" << v.x << "," << v.y << ")";
}

struct Segment {
    Vector p1, p2;

    Segment(const Vector& pp1, const Vector& pp2) : p1(pp1), p2(pp2) {}
    Segment(f64 x1, f64 y1, f64 x2, f64 y2) : p1(Vector(x1,y1)), p2(Vector(x2,y2)) {}

    Vector vec() const { return p2 - p1; }

    f64 norm() const { return vec().norm(); }
    f64 abs() const { return vec().abs(); }
};

ostream& operator<<(ostream& out, const Segment& seg) {
    out << "Segment(";
    out << "(" << seg.p1.x << "," << seg.p1.y << ")";
    out << ",";
    out << "(" << seg.p2.x << "," << seg.p2.y << ")";
    out << ")";
    return out;
}

struct Line {
    Vector p1, p2;

    Line(const Vector& pp1, const Vector& pp2) : p1(pp1), p2(pp2) {}
    Line(f64 x1, f64 y1, f64 x2, f64 y2) : p1(Vector(x1,y1)), p2(Vector(x2,y2)) {}
    Line(const Segment& seg) : p1(seg.p1), p2(seg.p2) {}

    Vector vec() const { return p2 - p1; }
};

ostream& operator<<(ostream& out, const Line& line) {
    out << "Line(";
    out << "(" << line.p1.x << "," << line.p1.y << ")";
    out << ",";
    out << "(" << line.p2.x << "," << line.p2.y << ")";
    out << ")";
    return out;
}

struct Circle {
    Vector c;
    f64 r;

    Circle(const Vector& cc, f64 rr) : c(cc), r(rr) {}
};

ostream& operator<<(ostream& out, const Circle& cir) {
    out << "Circle(";
    out << "(" << cir.c.x << "," << cir.c.y << ")";
    out << ",";
    out << cir.r;
    out << ")";
    return out;
}

f64 geo_dot(const Vector& lhs, const Vector& rhs) {
    return lhs.x*rhs.x + lhs.y*rhs.y;
}

f64 geo_cross(const Vector& lhs, const Vector& rhs) {
    return lhs.x*rhs.y - lhs.y*rhs.x;
}

Vector geo_rotate(const Vector& v, f64 t) {
    return Vector(v.x*cos(t)-v.y*sin(t), v.x*sin(t)+v.y*cos(t));
}

Vector geo_project(const Line& line, const Vector& p) {
    Vector v = line.vec();
    f64 r = geo_dot(p-line.p1, v) / v.norm();
    return line.p1 + r*v;
}

enum ABC {
    ABC_CCW        =  1,
    ABC_CW         = -1,
    ABC_ON_BACK    =  2,
    ABC_ON_FRONT   = -2,
    ABC_ON_SEGMENT =  0,
};

ABC geo_abc(const Vector& a, const Vector& b, const Vector& c) {
    Vector x = b - a;
    Vector y = c - a;
    f64 cross = geo_cross(x,y);
    if(cross > 0) return ABC_CCW;
    if(cross < 0) return ABC_CW;
    f64 dot = geo_dot(x,y);
    if(dot < 0) return ABC_ON_BACK;
    if(x.norm() < y.norm()) return ABC_ON_FRONT;
    return ABC_ON_SEGMENT;
}

bool geo_intersect(const Segment& x, const Segment& y) {
    return geo_abc(x.p1,x.p2,y.p1) * geo_abc(x.p1,x.p2,y.p2) <= 0 &&
           geo_abc(y.p1,y.p2,x.p1) * geo_abc(y.p1,y.p2,x.p2) <= 0;
}

bool geo_intersect(const Circle& cir1, const Circle& cir2) {
    return (cir1.c-cir2.c).norm() <= pow(cir1.r+cir2.r,2);
}

f64 geo_distance(const Line& line, const Vector& p) {
    Vector v = line.vec();
    return fabs(geo_cross(v,p-line.p1)) / v.abs();
}

f64 geo_distance(const Segment& seg, const Vector& p) {
    if(geo_dot( seg.vec(), p-seg.p1) < 0) return (p-seg.p1).abs();
    if(geo_dot(-seg.vec(), p-seg.p2) < 0) return (p-seg.p2).abs();
    return geo_distance(Line(seg), p);
}

f64 geo_distance(const Segment& seg1, const Segment& seg2) {
    if(geo_intersect(seg1,seg2)) return 0;
    return min({
        geo_distance(seg1, seg2.p1),
        geo_distance(seg1, seg2.p2),
        geo_distance(seg2, seg1.p1),
        geo_distance(seg2, seg1.p2),
    });
}

bool geo_intersect(const Circle& cir, const Line& line) {
    return geo_distance(line, cir.c) <= cir.r;
}

Vector geo_crosspoint(const Segment& x, const Segment& y) {
    Line ly = Line(y);
    f64 d1 = geo_distance(ly, x.p1);
    f64 d2 = geo_distance(ly, x.p2);
    return x.p1 + (d1/(d1+d2))*x.vec();
}

// 接する場合も同じ座標2つを返す
vector<Vector> geo_crosspoints(const Circle& cir, const Line& line) {
    if(!geo_intersect(cir,line)) return {};
    Vector p = geo_project(line, cir.c);
    Vector e = line.vec().unit();
    f64 t = sqrt(cir.r*cir.r - (p-cir.c).norm());
    return { p+t*e, p-t*e };
}

// 接する場合も同じ座標2つを返す
vector<Vector> geo_crosspoints(const Circle& cir1, const Circle& cir2) {
    if(!geo_intersect(cir1,cir2)) return {};
    Vector v = cir2.c - cir1.c;
    f64 d = v.abs();
    f64 t = acos((cir1.r*cir1.r + d*d - cir2.r*cir2.r) / (2*cir1.r*d));
    return {
        cir1.c + geo_rotate(v, t) / d * cir1.r,
        cir1.c + geo_rotate(v,-t) / d * cir1.r,
    };
}

void RD(Vector& v) {
    RD(v.x);
    RD(v.y);
}

//--------------------------------------------------------------------

struct Polygon {
    vector<Vector> ps;

    explicit Polygon(const vector<Vector>& ps_arg) : ps(ps_arg) {}

    f64 area_2x() const {
        i64 n = SIZE(ps);
        if(n < 3) return 0;
        f64 sum = geo_cross(ps.back(), ps.front());
        REP(i, n-1) {
            sum += geo_cross(ps[i], ps[i+1]);
        }
        return fabs(sum);
    }
    f64 area() const { return area_2x() / 2; }
};

void solve() {
    i64 N; RD(N);
    vector<Vector> ps; RD(ps, N);
    Polygon poly(ps);

    f64 ans = poly.area();

    cout << fixed << setprecision(1);
    PRINTLN(ans);
}

signed main() {
    

    solve();

    return 0;
}



Measured memory_bytes: 3324