#include<stdio.h>
struct Struct
{
	double x,y;
} a[100005];
int main()
{
	freopen("aria.in","r",stdin);
	freopen("aria.out","w",stdout);
	int n;
	double S=0;
	scanf("%d",&n);
	for(int i=1;i<=n;i++)
		scanf("%lf%lf",&a[i].x,&a[i].y);
	a[0]=a[n];
	for(int i=0;i<n;i++)
		S=S+a[i].x*a[i+1].y-a[i].y*a[i+1].x;
	printf("%.5lf\n",0.5*S);
	return 0;
}