"use strict";
const main = arg => {
  let Nums = arg.split(" ").map(n => n*1);
  let ans = 0;
  ans = Nums[0]+Nums[1]%2 !== 1 ? (Nums[0]+Nums[1])/2 : "IMPOSSIBLE"; 
  console.log(ans);
}

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