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

int main(){
    int N; cin >> N;
    vector<int> x(N), y(N);
    for(int i = 0; i < N; ++i) cin >> x[i] >> y[i];
    int ans = 0;
    for(int i = 1; i < N - 1; ++i){
        ans += abs((x[i] - x[0]) * (y[i + 1] - y[0]) - (y[i] - y[0]) * (x[i + 1] - x[0]));
    }
    cout << ans << endl;
}