#include <stdio.h>
#include <math.h>

double distanta(double x1 , double y1 , double x2 , double y2){
	//teorema lui pitagora
	double l,arg;
	arg=(x1-x2)*(x1-x2)+(y1-y2)*(y1-y2);
	l=sqrt(arg);
	return l;
}

double arie(double x , double y , double z){
	//formula lui heron
	double a,p,arg;
	p=(x+y+z)/2;
	a=sqrt(p*(p-x)*(p-y)*(p-z));
	return a;
}

int main()
{
	double N,x,y,x1,y1,x2,y2,l1,l2,l3,sum=0,i;
	scanf("%lf %lf %lf" , &N , &x , &y);
	scanf("%lf %lf" , &x2 , &y2);
	for(i=1 ; i<N-1 ; i++){
		x1=x2;
		y1=y2;
		scanf("%lf %lf" , &x2 , &y2);
		l1=distanta(x , y , x1 , y1);
		l2=distanta(x , y , x2 , y2);
		l3=distanta(x1 , y1 , x2 , y2);
		sum+=arie(l1 , l2 , l3);
	}
	printf("%lf\n", sum);
	return 0;
}