#include <stdio.h>
int main() {int N,i,RB[51][2],TT=1;while (1) {scanf("%d",&N);if (!N) return 0;for (i=0;i<N;scanf("%d %d",&RB[i][0],&RB[i][1]),i++);int ans=0;for (i=0;i<N;i++) ans+=RB[i][0]*RB[(i+1)%N][1]-RB[i][1]*RB[(i+1)%N][0];if (ans<0) ans=-ans;printf("%d %.1lf\n",TT++,ans/2.00);}}