function Main(input) {
  var A = parseInt(input.split(" ")[0]);
  var B = parseInt(input.split(" ")[1]);
  
  if((A % 2 == 0 && B % 2 == 1) && (A % 2 == 1 && B % 2 == 0)){
    console.log("IMPOSSIBLE") ;
    return;
  }else{
    console.log((A + B) / 2);
  }
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));
