#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;

int N, X[33], Y[33], sum = 0;

int main() {
  cin >> N;
  for (int i = 1; i <= N; i++) {
    cin >> X[i] >> Y[i];
    if (i != 1) {
      X[i] -= X[1];
      Y[i] -= Y[1];
    }
  }
  X[1] = 0, Y[1] = 0;
  for (int i = 2; i <= N - 1; i++) {
    sum += X[i] * Y[i + 1] - X[i + 1] * Y[i];
  }
  cout << sum << endl;
  return 0;
}