hdu 2036

#include <iostream>
#include <cstdio>
using namespace std;
struct line
{
    int x,y;
}data[105];
double Area(line p,line q)//面积
{
    return p.x*q.y-p.y*q.x;
}
int main()
{
    int m;
    while(cin>>m,m)
    {
        for(int i=0; i<m; i++)
         cin>>data[i].x>>data[i].y;
        double ans=Area(data[m-1],data[0]);
        for(int i=1; i<m; i++)
            ans+=Area(data[i-1],data[i]);
        printf("%.1lf\n",0.5*ans);
    }
    return 0;
}