#include <fstream>
#include <iomanip>
#include <vector>

using namespace std;

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

int main() {
    int n;
    cin >> n;
    vector<pair<double, double>> pct(n);
    for (int i = 0; i < n; i++) {
        cin >> pct[i].first >> pct[i].second;
    }
    double sum = 0;
    for (int i = 0; i < n; i++) {
        int j = (i + 1) % n;
        sum += (pct[i].first * pct[j].second - pct[j].first * pct[i].second);
    }
    sum = abs(sum) / 2.0;
    cout << setprecision(5) << fixed << sum;
    return 0;
}