#include<stdio.h>

double area(double x0, double y0, double x1,double y1, double x2,double y2){

return (x0*y1+x2*y0+x1*y2-x2*y1-x0*y2-x1*y0)/2;

}

int main(){

int n;

while(scanf("%d",&n) == 1&&n){

double x[109], y[109];

double sum = 0;

for(int i = 1; i<= n; i++){

scanf("%lf%lf",&x[i],&y[i]);

}

for(int i = 2;i < n; i++){

sum += area(x[1],y[1],x[i],y[i],x[i+1],y[i+1]);

}

printf("%.1lf\n",sum);

}

return 0;

}