#include <cstdio>
#include <bits/stdc++.h>

using namespace std;

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);

    double x0, y0, x1, y1, x2 = 0, y2 = 0, area = 0;

    int N; cin >> N >> x0 >> y0;

    x1 = x0; y1 = y0;

    while(--N)
    {
        cin >> x2 >> y2;

        area += x1 * y2 - x2 * y1;

        x1 = x2; y1 = y2;
    }

    area += x1 * y0 - x0 * y1;

    cout << setprecision(5) << fixed << area / 2.0;

    return 0;
}