#include <complex>
#include <cmath>
#include <iostream>
using namespace std;
typedef complex<double> xy_t;

xy_t P[110];

double cross_product(xy_t a, xy_t b) { return (conj(a)*b).imag(); }

int main() {
  int N = 0;
  double x, y;
  cin >> N;
  for (int i=0; i<N; i++) {
    cin >> x >> y;
    P[i] = xy_t(x, y);
  }

  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;
  }

  printf("%.1f\n", abs(sum));
}
