#include <iostream>
#include <cstdio>
#define NMAX 100000

using namespace std;

struct point {
  long double x, y;
} v[NMAX + 5];

int n;
long double area = 0;

int main() {
  freopen("aria.in", "r", stdin);
  freopen("aria.out", "w", stdout);
  double d1, d2;

  scanf("%d", &n);
  for(int i = 1; i <= n; i++)
    cin >> v[i].x >> v[i].y;
  v[n + 1].x = v[1].x;
  v[n + 1].y = v[1].y;
  for(int i = 2; i <= n; i++)
    area += (v[i].x * v[i + 1].y - v[i].y * v[i + 1].x);

  if(area < 0)
    cout << -area / 2;
  else
    cout << area / 2;

  return 0;
}