var input = require('fs').readFileSync('/dev/stdin', 'utf8');
var arr=(input.trim()).split("\n");
var cnt=1;
while(true){
   var n=arr.shift()-0;
   if(n==0)break;
   var xy=[];
   while(n--)xy.push(arr.shift().split(" ").map(Number));
   var S=0;
   for(var i=0;i<xy.length-1;i++){
      S+=(xy[i][0]-xy[i+1][0])*(xy[i][1]+xy[i+1][1]);
   }
   S+=(xy[xy.length-1][0]-xy[0][0])*(xy[xy.length-1][1]+xy[0][1]);
   console.log(cnt+" "+Math.abs(S/2).toFixed(1));
   arr.shift();
   cnt++;
}