#include<cstdio>
using namespace std;
int n;
struct punct
{
	double x,y;
}v[100005];
double aria;
void citire()
{
	scanf("%d",&n);
	for(int i=1;i<=n;i++)
		scanf("%lf%lf",&v[i].x,&v[i].y);
	v[++n].x=v[1].x;
	v[n].y=v[1].y;
}
int main()
{
	freopen("aria.in","r",stdin);
	freopen("aria.out","w",stdout);
	
	citire();
	for(int i=2;i<=n+1;i++)
		aria+=v[i].x*v[i-1].y-v[i-1].x*v[i].y;
	if(aria<0) aria*=-1;
	printf("%.5lf\n",aria/2);
	return 0;
}