#include<stdio.h> 
#include<stdlib.h>
FILE * Fin=fopen("aria.in","r"), *Fout=fopen("aria.out","w");
int main ()
{
	int Aria1,x,x1,y1,y,AriaTotal=0,n,i,j,a[100001][2];
	fscanf(Fin,"%d%d%d",&n,&x,&y);x1=x;y1=y;
	for(i=1;i<n;++i)
	{
		for(j=0;j<2;++j)
			fscanf(Fin,"%d",&a[i][j]);
		Aria1=abs((x*a[i][1]-y*a[i][0])/2);
		x=a[i][0]; y=a[i][1];
		AriaTotal+=Aria1;
	}
	AriaTotal+=abs((x1*a[i-1][1]-y1*a[i-1][0])/2);
	fprintf(Fout,"%d",AriaTotal);
	return 0;
}