#include<stdio.h>
#include<stdlib.h>
int main()
{
int n;
while(scanf("%d",&n)!=EOF&&n)
{ double sum=0,x[100],y[100];
for(int i=0;i<n;i++)scanf("%lf %lf",&x[i],&y[i]);
for(int i=1;i<n-1;i++)
{
double s=x[0]*y[i]-x[i]*y[0]+x[i]*y[i+1]-x[i+1]*y[i]+x[i+1]*y[0]-x[0]*y[i+1];
sum += 0.5*s;
}
printf("%.1lf\n",sum);
}
return 0;
}