'use strict'
const main = input => {
  input = input[0].split(' ').map(val => parseInt(val));
  const a = input[0], b = input[1], m = b+(a-b)/2;
  console.log(m == Math.round(m) ? m : 'IMPOSSIBLE');
  
  
  
}
main(require("fs").readFileSync("/dev/stdin", "utf8").split('\n'));