#include <stdio.h>

int main(void)
{
    int n;
    double ans = 0;
    scanf("%d", &n);
    
    double x1, y1, xp, yp, x, y;
    scanf("%lf%lf", &x1, &y1);
    xp = x1;
    yp = y1;

    for(int i = 1; i < n; i++) {
        scanf("%lf%lf", &x, &y);

        ans += xp * y - x * yp;

        xp = x;
        yp = y;
    }

    ans += x * y1 - x1 * y;

    ans /= 2;

    printf("%.5lf\n", ans);

    return 0;

}