#include <cstdio>
#include <cmath>
using namespace std;
double x[100005], y[100005];
int main(){
  freopen("aria.in", "r", stdin);
  freopen("aria.out", "w", stdout);
  int n, i;
  double arie = 0;
  scanf("%d", &n);
  for (i = 1; i <= n; i ++)
    scanf("%lf%lf", &x[i], &y[i]);
  x[n + 1] = x[1];
  y[n + 1] = y[1];
  for (i = 1; i <= n; i ++)
    arie += (x[i] * y[i + 1] - x[i + 1] * y[i]);
  printf("%lf", fabs(arie / 2.0));
  return 0;
}