#include <cstdio>
using namespace std;
int main()
{
	double s=0, x1, y1, xi, yi, xj, yj;
	int n, i;
	freopen("aria.in", "r", stdin);
	freopen("aria.out", "w", stdout);
	scanf("%d%lf%lf", &n, &x1, &y1);
	xi=x1; yi=y1;
	for(i=2; i<=n; i++){
		scanf("%lf%lf", &xj, &yj);
		s = s + xi*yj-xj*yi;
		xi = xj; yj = yi;
	}
	s = s + xj*y1-x1*yj;
	printf("%lf", s);
}