#include <cstdio>
#include <algorithm>
using namespace std;
struct Point{
  int x, y;
};
double arie(Point A, Point B, Point C){
  return 1. * (A.x * (B.y - C.y) + B.x * (C.y - A.y) + C.x * (A.y - B.y)) / 2.0;
}
Point v[100005];
int main(){
  freopen("aria.in", "r", stdin);
  freopen("aria.out", "w", stdout);
  int n;
  scanf("%d", &n);
  for (int i = 1; i <= n; ++i)
    scanf("%d%d", &v[i].x, &v[i].y);
  double aria = 0;
  for (int i = 2; i < n; ++i)
    aria += arie(v[1], v[i], v[i + 1]);
  printf("%.5f", fabs(aria));
  return 0;
}