/*
    Keep It Simple!
*/

#include<stdio.h>
int n,x[100001],y[100001];
long long a;

int main()
{
	
	freopen("aria.in","r",stdin);
	freopen("aria.out","w",stdout);

    scanf("%d",&n);
	for(int i=1;i<=n;i++)
		scanf("%d %d",&x[i],&y[i]);
	x[0] = x[n];
	y[0] = y[n];
	for(int i=0;i<n;i++)
		a+= x[i]*y[i+1] - y[i] * x[i+1] ;
	if(a<0) a = -a;
	a/=2;
	printf("%lld",a);
}