#line 1 "main.cpp"
#include <iostream>
#include <cmath>
#include <vector>

using namespace std;

void solve() {
    int n;
    cin >> n;

    vector<pair<int, int>> ps(n);
    for (auto& [x, y] : ps) cin >> x >> y;

    int ans = 0;
    for (int i = 0; i < n; ++i) {
        auto [x1, y1] = ps[i];
        auto [x2, y2] = ps[(i + 1) % n];

        ans += x1 * y2 - x2 * y1;
    }

    cout << abs(ans) << "\n";
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    solve();

    return 0;
}