#include <cstdio>
#include <cmath>
using namespace std;
int n,i;
double arie,vecx[100002],vecy[100002];
int main()
{
	freopen("aria.in","r",stdin);
	freopen("aria.out","w",stdout);
	scanf("%ld",&n);
	for(i=0;i<n;i++){scanf ("%lf %lf", &vecx[i], &vecy[i]);}
	vecx[n]=vecx[0];
	vecy[n]=vecy[0];
	for(i=0;i<n;i++){arie+=vecx[i]*vecy[i+1]-vecx[i+1]*vecy[i];}
	printf("%lf",fabs(arie/2));
return 0;
}