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