#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    using Point = pair<ll,ll>;
    int N; cin >> N;
    vector<Point> P(N);
    for(auto &[x, y] : P) cin >> x >> y;
    ll ans = 0;
    auto cross = [&](Point A, Point B) {
        return A.first * B.second - A.second * B.first;
    };
    rep(i,N) ans += cross(P[i], P[(i + 1) % N]);
    cout << ans << endl;
}