#include <cstdio>

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;

    for(int i = 2; i <= N; ++i)
    {
        scanf("%lf %lf", &x2, &y2);

        area += x1 * y2 - x2 * y1;

        x1 = x2; y1 = y2;
    }

    area += x2 * y0 - x0 * y2;

    printf("%.10lf", area / 2);

    return 0;
}