#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, e) for (int i = (int)(s); i < (int)(e); ++i)
#define all(a) (a).begin(),(a).end()

int main() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    
    int N;
    cin >> N;
    vector<ll> X(N), Y(N);
    rep(i, 0, N) cin >> X[i] >> Y[i];
    
    ll ans = 0;
    rep(i, 1, N - 1) {
        ans += abs((X[0] - X[i + 1])*(Y[i] - Y[i + 1]) - (X[i] - X[i + 1])*(Y[0] - Y[i + 1]));
    }
    
    cout << ans << '\n';
}