#include <stdio.h>
#include<math.h>
#include<stdlib.h>
#include<string.h>
#include <iostream>
#include<algorithm>
#define max_v 105
int x[max_v],y[max_v];
int main()
{
    int n;
    while(~scanf("%d",&n))
    {
        if(n==0)
            break;
        double sum=0;
        for(int i=0;i<n;i++)
            scanf("%d %d",&x[i],&y[i]);
        for(int i=0;i<n-1;i++)
        {
            sum+=x[i]*y[i+1]-x[i+1]*y[i];
        }
        sum+=x[n-1]*y[0]-x[0]*y[n-1];
        printf("%0.1lf\n",sum/2.0);
    }
    return 0;
}