function Main(input) {
  input = input.split("\n");
  tmp = input[0].split(" ");
  A = Number(tmp[0]);
  B = Number(tmp[1]);
  if(A%2!=B%2){
    console.log("IMPOSSIBLE");
  }else{
    console.log((A+B)/2);
  }
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));