#include<bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double lf;
typedef pair<int, int> pii;

const int MAXN = 5;
const int MOD = 1e9+7;

const lf eps = 1e-9;
typedef double T;
struct pt {
  T x, y;
  pt operator + (pt p) { return {x+p.x, y+p.y}; }
  pt operator - (pt p) { return {x-p.x, y-p.y}; }
  pt operator * (pt p) { return {x*p.x-y*p.y, x*p.y+y*p.x}; }
  pt operator * (T d) { return {x*d, y*d}; }
  pt operator / (T d) { return {x/d, y/d}; } /// only for floating point
  bool operator == (pt b) { return x == b.x && y == b.y; }
  bool operator != (pt b) { return !(*this == b); }
  bool operator < (const pt &o) const { return y < o.y || (y == o.y && x < o.x); }
  bool operator > (const pt &o) const { return y > o.y || (y == o.y && x > o.x); }
};
int cmp (lf a, lf b) { return (a + eps < b ? -1 :(b + eps < a ? 1 : 0)); }
/** Already in complex **/
T norm(pt a) { return a.x*a.x + a.y*a.y; }
T abs(pt a) { return sqrt(norm(a)); }
T arg(pt a) { return atan2(a.y, a.x); }
ostream& operator << (ostream& os, pt &p) {
  return os << "("<< p.x << "," << p.y << ")";
}
/***/
istream &operator >> (istream &in, pt &p) {
    T x, y; in >> x >> y;
    p = {x, y};
    return in;
}
T dot(pt a, pt b) { return a.x*b.x + a.y*b.y; }
T cross(pt a, pt b) { return a.x*b.y - a.y*b.x; }
T orient(pt a, pt b, pt c) { return cross(b-a,c-a); }
//pt rot(pt p, lf a) { return {p.x*cos(a) - p.y*sin(a), p.x*sin(a) + p.y*cos(a)}; }
//pt rot(pt p, double a) { return p * polar(1.0, a); } /// for complex
//pt rotate_to_b(pt a, pt b, lf ang) { return rot(a-b, ang)+b; }
pt rot90ccw(pt p) { return {-p.y, p.x}; }
pt rot90cw(pt p) { return {p.y, -p.x}; }
pt translate(pt p, pt v) { return p+v; }
pt scale(pt p, double f, pt c) { return c + (p-c)*f; }
bool are_perp(pt v, pt w) { return dot(v,w) == 0; }
int sign(T x) { return (T(0) < x) - (x < T(0)); }
pt unit(pt a) { return a/abs(a); }

bool in_angle(pt a, pt b, pt c, pt x) {
  assert(orient(a,b,c) != 0);
  if (orient(a,b,c) < 0) swap(b,c);
  return orient(a,b,x) >= 0 && orient(a,c,x) <= 0;
}

//lf angle(pt a, pt b) { return acos(max(-1.0, min(1.0, dot(a,b)/abs(a)/abs(b)))); }
//lf angle(pt a, pt b) { return atan2(cross(a, b), dot(a, b)); }
/// returns vector to transform points
pt get_linear_transformation(pt p, pt q, pt r, pt fp, pt fq) {
  pt pq = q-p, num{cross(pq, fq-fp), dot(pq, fq-fp)};
  return fp + pt{cross(r-p, num), dot(r-p, num)} / norm(pq);
}

bool half(pt p) { /// true if is in (0, 180]
  assert(p.x != 0 || p.y != 0); /// the argument of (0,0) is undefined
  return p.y > 0 || (p.y == 0 && p.x < 0);
}
bool half_from(pt p, pt v = {1, 0}) {
  return cross(v,p) < 0 || (cross(v,p) == 0 && dot(v,p) < 0);
}
bool polar_cmp(const pt &a, const pt &b) {
  return make_tuple(half(a), 0) < make_tuple(half(b), cross(a,b));
}

struct line {
  pt v; T c;
  line(pt v, T c) : v(v), c(c) {}
  line(T a, T b, T c) : v({b,-a}), c(c) {}
  line(pt p, pt q) : v(q-p), c(cross(v,p)) {}
  T side(pt p) { return cross(v,p)-c; }
  lf dist(pt p) { return abs(side(p)) / abs(v); }
  lf sq_dist(pt p) { return side(p)*side(p) / (lf)norm(v); }
  line perp_through(pt p) { return {p, p + rot90ccw(v)}; }
  bool cmp_proj(pt p, pt q) { return dot(v,p) < dot(v,q); }
  line translate(pt t) { return {v, c + cross(v,t)}; }
  line shift_left(double d) { return {v, c + d*abs(v)}; }
  pt proj(pt p) { return p - rot90ccw(v)*side(p)/norm(v); }
  pt refl(pt p) { return p - rot90ccw(v)*2*side(p)/norm(v); }
};

bool inter_ll(line l1, line l2, pt &out) {
  T d = cross(l1.v, l2.v);
  if (d == 0) return false;
  out = (l2.v*l1.c - l1.v*l2.c) / d;
  return true;
}
/*line bisector(line l1, line l2, bool interior) {
  assert(cross(l1.v, l2.v) != 0); /// l1 and l2 cannot be parallel!
  lf sign = interior ? 1 : -1;
  return {l2.v/abs(l2.v) + l1.v/abs(l1.v) * sign,
          l2.c/abs(l2.v) + l1.c/abs(l1.v) * sign};
}*/

bool in_disk(pt a, pt b, pt p) {
  return dot(a-p, b-p) <= 0;
}
bool on_segment(pt a, pt b, pt p) {
  return orient(a,b,p) == 0 && in_disk(a,b,p);
}
bool proper_inter(pt a, pt b, pt c, pt d, pt &out) {
  T oa = orient(c,d,a),
  ob = orient(c,d,b),
  oc = orient(a,b,c),
  od = orient(a,b,d);
  /// Proper intersection exists iff opposite signs
  if (oa*ob < 0 && oc*od < 0) {
    out = (a*ob - b*oa) / (ob-oa);
    return true;
  }
  return false;
}
set<pt> inter_ss(pt a, pt b, pt c, pt d) {
  pt out;
  if (proper_inter(a,b,c,d,out)) return {out};
  set<pt> s;
  if (on_segment(c,d,a)) s.insert(a);
  if (on_segment(c,d,b)) s.insert(b);
  if (on_segment(a,b,c)) s.insert(c);
  if (on_segment(a,b,d)) s.insert(d);
  return s;
}
lf pt_to_seg(pt a, pt b, pt p) {
  if(a != b) {
    line l(a,b);
    if (l.cmp_proj(a,p) && l.cmp_proj(p,b)) /// if closest to  projection
      return l.dist(p); /// output distance to line
  }
  return min(abs(p-a), abs(p-b)); /// otherwise distance to A or B
}
lf set_to_seg(pt a, pt b, pt c, pt d) {
  pt dummy;
  if (proper_inter(a,b,c,d,dummy)) return 0;
  return min({pt_to_seg(a,b,c), pt_to_seg(a,b,d),
              pt_to_seg(c,d,a), pt_to_seg(c,d,b)});
}

enum {IN, OUT, ON};
struct polygon {
  vector<pt> p;
  polygon(int n) : p(n) {}
  int top = -1, bottom = -1;
  void delete_repetead() {
    vector<pt> aux;
    sort(p.begin(), p.end());
    for(pt &i : p)
      if(aux.empty() || aux.back() != i)
        aux.push_back(i);
    p.swap(aux);
  }
  bool is_convex() {
    bool pos = 0, neg = 0;
    for (int i = 0, n = p.size(); i < n; i++) {
      int o = orient(p[i], p[(i+1)%n], p[(i+2)%n]);
      if (o > 0) pos = 1;
      if (o < 0) neg = 1;
    }
    return !(pos && neg);
  }
  lf area() {
    lf ans = 0;
    for (int i = 0, n = p.size(); i < n; i++)
      ans += cross(p[i], p[(i+1)%n]);
    return abs(ans) / 2.0;
  }
  lf perimeter() {
    lf per = 0;
    for(int i = 0, n = p.size(); i < n; i++)
      per += abs(p[i] - p[(i+1)%n]);
    return per;
  }
  bool above(pt a, pt p) { return p.y >= a.y; }
  bool crosses_ray(pt a, pt p, pt q) {
    return (above(a,q)-above(a,p))*orient(a,p,q) > 0;
  }
  int in_polygon(pt a) {
    int crosses = 0;
    for(int i = 0, n = p.size(); i < n; i++) {
      if(on_segment(p[i], p[(i+1)%n], a)) return ON;
      crosses += crosses_ray(a, p[i], p[(i+1)%n]);
    }
    return (crosses&1 ? IN : OUT);
  }
  void normalize() { /// polygon is CCW
    bottom = min_element(p.begin(), p.end()) - p.begin();
    vector<pt> tmp(p.begin()+bottom, p.end());
    tmp.insert(tmp.end(), p.begin(), p.begin()+bottom);
    p.swap(tmp);
    bottom = 0;
    top = max_element(p.begin(), p.end()) - p.begin();
  }
  int in_convex(pt a) {
    assert(bottom == 0 && top != -1);
    if(a < p[0] || a > p[top]) return OUT;
    T orientation = orient(p[0], p[top], a);
    if(orientation == 0) {
      if(a == p[0] || a == p[top]) return ON;
      return top == 1 || top + 1 == p.size() ? ON : IN;
    } else if (orientation < 0) {
      auto it = lower_bound(p.begin()+1, p.begin()+top, a);
      T d = orient(*prev(it), a, *it);
      return d < 0 ? IN : (d > 0 ? OUT: ON);
    }
    else {
      auto it = upper_bound(p.rbegin(), p.rend()-top-1, a);
      T d = orient(*it, a, it == p.rbegin() ? p[0] : *prev(it));
      return d < 0 ? IN : (d > 0 ? OUT: ON);
    }
  }
  polygon cut(pt a, pt b) {
    line l(a, b);
    polygon new_polygon(0);
    for(int i = 0, n = p.size(); i < n; ++i) {
      pt c = p[i], d = p[(i+1)%n];
      lf abc = cross(b-a, c-a), abd = cross(b-a, d-a);
      if(abc >= 0) new_polygon.p.push_back(c);
      if(abc*abd < 0) {
        pt out; inter_ll(l, line(c, d), out);
        new_polygon.p.push_back(out);
      }
    }
    return new_polygon;
  }
  void convex_hull() {
    sort(p.begin(), p.end());
    vector<pt> ch;
    ch.reserve(p.size()+1);
    for(int it = 0; it < 2; it++) {
      int start = ch.size();
      for(auto &a : p) {
        /// if colineal are needed, use < and remove repeated points
        while(ch.size() >= start+2 && orient(ch[ch.size()-2], ch.back(), a) <= 0)
          ch.pop_back();
        ch.push_back(a);
      }
      ch.pop_back();
      reverse(p.begin(), p.end());
    }
    if(ch.size() == 2 && ch[0] == ch[1]) ch.pop_back();
    /// be careful with CH of size < 3
    p.swap(ch);
  }
  ll pick() {
    ll boundary = 0;
    for(int i = 0, n = p.size(); i < n; i++) {
      int j = (i+1 == n ? 0 : i+1);
      boundary += __gcd((ll)abs(p[i].x - p[j].x), (ll)abs(p[i].y - p[j].y));
    }
    return area() + 1 - boundary/2;
  }
  pt& operator[] (int i){ return p[i]; }
};

int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);

  #ifdef LOCAL
      freopen("input.txt", "r", stdin);
      freopen("output.txt", "w", stdout);
  #else
      #define endl '\n'
  #endif // LOCAL

  int n; cin >> n;
  polygon pol(n);
  for(int i = 0; i < n; i++) {
    cin >> pol[i];
  }
  cout << fixed << setprecision(1);
  cout << pol.area() << endl;
  return 0;
}



Measured memory_bytes: 3292