#include <stdio.h>
#define MAX 100005

int n, i;
double xi, yi, x1, y1, x2, y2, A;

int main(){
	freopen("aria.in", "r", stdin);
	freopen("aria.out", "w", stdout);
	scanf("%d", &n);
	scanf("%lf %lf", &xi, &yi);
	x1 = xi, y1 = yi;
	for(i = 1; i < n; i++){
		scanf("%lf%lf", &x2, &y2);
		A += (x1 * y2 - x2 * y1) / 2;
		x1 = x2, y1 = y2;
	}
	scanf("%lf%lf", &x1, &y1);
	A += (x1 * yi - xi * y1) / 2;
	printf("%lf\n", A);
	return 0;
}