#include <bits/stdc++.h>

using namespace std;

int main() {
    ifstream cin ("aria.in");
    ofstream cout ("aria.out");

    int n; cin >> n;
    vector < pair <double, double> > v(n + 1);

    for (auto& it : v)
       cin >> it.first >> it.second;

    v[n] = v[0];
    double Seeked = 0;
    for (int i = 0; i < n; ++i)
        Seeked += v[i].first * v[i + 1].second - v[i + 1].first * v[i].second;

    cout << fixed << setprecision(5) << Seeked / 2.0;
    return 0;
}