#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; scanf("%d %Lf %Lf", &N, &x0, &y0);

    x1 = x0; y1 = y0;

    while(--N)
    {
        scanf("%Lf %Lf", &x2, &y2);

        area += x1 * y2 - x2 * y1;

        x1 = x2; y1 = y2;
    }

    area += x1 * y0 - x0 * y1;

    printf("%Lf", area / 2);

    return 0;
}