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

ll calc(ll a, ll b, ll c, ll d, ll e, ll f) {
    ll A = c - a, B = d - b, C = e - a, D = f - b;
    return abs(A * D - B * C);
}

int main() {
    cin.tie(0);
    ios_base::sync_with_stdio(false);

    int n;
    cin >> n;
    vector<ll> x(n), y(n);
    rep(i, n) {
        cin >> x[i] >> y[i];
    }
    ll ans = 0;
    for(int i = 1; i < n - 1; i++) {
        ans += calc(x[0], y[0], x[i], y[i], x[i + 1], y[i + 1]);
    }
    cout << ans << endl;
    return 0;
}