#include <stdio.h>
typedef struct{
    double x,y;
}point;
int main(){
    int n;
    point p[50];
    while(1){
        scanf("%d",&n);
        double s=0;
        if(n==0) return 0;
        for(int i=0;i<n;i++) scanf("%f %f",&p[i].x,&p[i].y);
        for(int i=1;i<n-1;i++){
            s+=(p[0].x*(p[i].y-p[i+1].y)+p[i].x*(p[i+1].y-p[0].y)+p[i+1].x*(p[0].y-p[i].y))/2;
        }
        printf("%.1f\n",-s);
    }
}