/* 
   Keep It Simple!
*/
#include<stdio.h>

int n;
float x[100001],y[100001];
double A;
int main()
{
	freopen("aria.in","r",stdin);
	freopen("aria.out","w",stdout);
	
	scanf("%d",&n);
	for(int i=1;i<=n;i++)
		scanf("%f %f",&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] - x[i+1]*y[i]  );
	if(!A)
		A = -A;
	A/=2;
	printf("%.5f",A);
}