#include<stdio.h>
#include<stdlib.h>
#include <iostream>
using namespace std;
struct node
{
    double x;
    double y;
};
double get_area(node a,node b)
{
    return a.x*b.y-a.y*b.x;
}
int main()
{
    int n;
    double sum;
    while(cin>>n&&n)
    {
        sum=0;
        node poi[110];
      for(int i=0;i<n;i++)
      {
        cin>>poi[i].x>>poi[i].y;
      }
      for(int i=1;i<n;i++)
      {
        sum+=get_area(poi[i-1],poi[i]);
      }
      sum-=get_area(poi[0],poi[n-1]);
      printf("%.1f\n",sum/2);
    }
    return 0;
}