#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  int n;
  cin >> n;
  vector<pair<int, int>> v(n);
  rep(i, n) cin >> v[i].first >> v[i].second;
  ll ans = 0;
  rep(i, n - 1) {
    ll x1 = v[i].first - v[0].first, y1 = v[i].second - v[0].second;
    ll x2 = v[i + 1].first - v[0].first, y2 = v[i + 1].second - v[0].second;
    ans += x1 * y2 - x2 * y1;
  }
  cout << ans << endl;
  return 0;
}