#include <stdio.h>

struct point{
		double x,y;
	}a,b,c;

double aria(struct point a,struct point b)
{
	return (0.5*a.x)*b.y - (0.5*b.x)*a.y;
}



int main()
{
	int n;
	double surface=0;

	if(n<3){
	printf("0");
	return 0;
	}

	scanf("%d",&n);

	//scanf("%lf%lf",&a.x, &a.y);
	//scanf("%lf%lf",&b.x, &b.y);

	scanf("%lf%lf",&a.x, &a.y);
	c = a;

	for(int i=2;i<=n;i++)
	{
		scanf("%lf%lf",&b.x, &b.y);
		surface+=aria(a,b);
		//printf("\n%lf",aria(a,b) );
		a = b;

	}
	surface+=aria(b,c);
	

	printf("%lf\n",surface);


	

	return 0;
}