#include<stdio.h>

struct point
{
	int x,y;
}p,d;
int n;
int Aria;
int main()
{
	freopen("aria.in","r",stdin);
	freopen("aria.out","w",stdout);
	
	scanf("%d%d%d",&n,&p.x,&p.y);
	point first=p;
	for(int i=1;i<n;i++)
	{
		scanf("%d%d",&d.x,&d.y);
		Aria+=((p.x*d.y)-(d.x*p.y));
		p=d;
	}
	Aria+=((p.x*first.y)-(first.x*p.y));
	printf("%d",Aria/2);
}