#include<stdio.h>
int main()
{
	int i,n;
	float x[10000],y[10000],s;
	freopen( "aria.in", "r", stdin);
	freopen("aria.out", "w", stdout);
	scanf("%d",&n);
	for(i=1;i<=n;i++)
	{
		scanf("%f",&x[i]);
		scanf("%f",&y[i]);
	}
	x[n+1]=x[1];
	y[n+1]=y[1];
	s=0;
	for(i=1;i<=n;i++)
	s=s+x[i]*y[i+1]-x[i+1]*y[i];
    s=s/2;
    printf("%f",s);
    return 0;
}