function Main(input) {
  input = input.split("\n");
  const a = parseInt(input[0].split(" ")[0], 10);
  const b = parseInt(input[0].split(" ")[1], 10);
  const c = (a + b) / 2

  console.log((a + b) % 2 === 0 ? c : 'IMPOSSIBLE')
}


// *出力
// number
// console.log('%d', );
// string
// console.log('%s', );

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