"use strict"

const main = (input) => {
  const _input = input.split(" ");
  const A = parseInt(_input[0], 10);
  const B = parseInt(_input[1], 10);

  const sum = A + B;
  const div = Math.floor(sum / 2);
  const mod = sum % 2;

  if(mod != 0){
    console.log("IMPOSSIBLE");
    return;
  }

  console.log(div);
}

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