#include <bits/stdc++.h>

using namespace std;

int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");

    double A = 0;
    int N, x1, y1, x, y, xp, yp;
    f >> N >> x1 >> y1;
    xp = x1; yp = y1;
    for(int i = 2; i <= N; i ++) {
        f >> x >> y;
        A += (xp * y - yp * x);
        xp = x; yp = y;
    }
    A += (xp * x1 - yp * y1);
    g << setprecision(10) << A / 2 << "\n";
    return 0;
}