#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < n; i++)
using ll = long long;
int main() {
  ll N;
  cin >> N;
  vector<int> X(N), Y(N);
  rep(i, N) cin >> X[i] >> Y[i];
  ll ans = 0;
  for(int i = 0; i < N-1; i++) {
    ans += X[i]*Y[i+1]-X[i+1]*Y[i];
  }
  ans += X[N-1]*Y[0]-X[0]*Y[N-1];
  cout << abs(ans) << endl;
}