#include<stdio.h>

struct point
{
	float x,y;
}v[100005];
float A;
int n;
int main()
{
	freopen("aria.in","r",stdin);
	freopen("aria.out","w",stdout);
	A=0;
	scanf("%d",&n);

	for(int i=1;i<=n;i++)
		scanf("%f%f",&v[i].x,&v[i].y);
	v[0]=v[n];
	for(int i=0;i<n;i++)
		A+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
	printf("%.4f",A/2);
}