function Main(input){
var inputs = input.split(" ");
var A = new Number(inputs[0]);
var B = new Number(inputs[1]);
var K = (A+B)/2;
var jud = Number.isInteger(K);
if(jud == true){
  console.log(K);
}else{
  console.log("IMPOSSIBLE")
}
}
Main(require('fs').readFileSync('/dev/stdin','utf8'));