#include <bits/stdc++.h>
using namespace std;

int main() {
  int n; cin >> n;
  int a[35], b[35];
  for(int i = 0; i < n; ++i) cin >> a[i] >> b[i];
  int ans = 0;
  for(int i = 0; i < n; ++i) {
    int x1, y1, x2, y2;
    x1 = a[i];
    y1 = b[i];
    x2 = a[(i+1)%n];
    y2 = b[(i+1)%n];
    ans += x1 * y2 - x2 * y1;
  }
  cout << ans << endl;
}