#include <iostream>
#include <vector>
#include <cmath>
#include <iomanip>
using namespace std;
 
int main() {
    int n;
    cin >> n;
 
    vector<pair<double, double>> tacke(n);
    for (int i = 0; i < n; i++) {
        cin >> tacke[i].first >> tacke[i].second;
    }
 
    double zbir = 0;
    for (int i = 0; i < n; i++) {
        int j = (i + 1) % n;
        zbir += tacke[i].first * tacke[j].second - tacke[j].first * tacke[i].second;
    }
 
    double povrsina = fabs(zbir) / 2.0;
 
    cout << fixed << setprecision(1) << povrsina;
 
    return 0;
}

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