#include <stdio.h>

FILE *f,*s;

struct punct
{
	double x;
	double y;
};

punct v[100005];

int n,i;

double sum;

int main()
{
	f=fopen("aria.in","r");
	s=fopen("aria.out","w");
	
	fscanf(f,"%d",&n);
	
	for(i=1;i<=n;i++)
		fscanf(f,"%lf %lf",&v[i].x,&v[i].y);
	
	v[n+1]=v[1];
	
	for(i=1;i<=n;i++)
		sum+=(v[i].x*v[i+1].y)-(v[i+1].x*v[i].y);
	
	fprintf(s,"%lf",sum/2);
	
	fclose(s);
	
	return 0;
}