#include <cstdio>
using namespace std;
int n,i;
double x0,y0,x1,y1,x2,y2,arie;
int main()
{
	freopen("aria.in","r",stdin);
	freopen("aria.out","w",stdout);
	scanf("%ld",&n);
	scanf("%lf%lf",&x0,&y0);x1=x0;y1=y0;
	for(i=2;i<=n;i++){scanf("%lf%lf",&x2,&y2);arie+=x1*y2-x2*y1;x1=x2;y1=y2;}
	arie+=x1*y0-x0*y1;
	if(arie<0){arie*=-1.0;}
	arie/=2.0;
	printf("%lf",arie);
return 0;
}