#if 0

#endif

#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef complex<double> xy_t;

double dot_product(xy_t a, xy_t b) {
  return (conj(a)*b).real();
}
double cross_product(xy_t a, xy_t b) {
  return (conj(a)*b).imag();
}
xy_t projection(xy_t p, xy_t b) {
  return b*dot_product(p,b)/norm(b);
}

xy_t P[110];
int N;
int main() {
  cin >> N;
  int x, y;
  for (int i = 0; i < N; i++) {
    cin >> x >> y;
    P[i] = xy_t(x*1.0,y*1.0);
  }
  double sum = 0.0;
  for (int i = 0; i + 2 < N; ++i) {
    xy_t a = P[0], b = P[i+1], c = P[i+2];

    sum += cross_product(b-a, c-a)/2.0;
  }
  printf("%.1f\n", abs(sum));
}

