#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;

int main() {
    ifstream fin("aria.in");
    ofstream fout("aria.out");

    int n;
    fin >> n;

    long double a = 0.0, x1, y1, x2, y2, x_start, y_start;
    fin >> x_start >> y_start;
    y1 = y_start;

    for (int i = 1; i < n; i++) {
        fin >> x2 >> y2;
        a += (x1 * y2) - (x2 * y1);
        x1 = x2;
        y1 = y2;
    }

    a += (x1 * y_start) - (x_start * y1);
    fout << fixed << setprecision(5) << abs(a / 2.0) << endl;

    return 0;
}