"use strict"

const arg_str = require('fs').readFileSync('/dev/stdin', 'UTF-8')
const args= arg_str.split(' ')
const arg_1 = parseInt(args[0])
const arg_2 = parseInt(args[1])

const sum = arg_1 + arg_2
const div = Math.floor(sum / 2)
const mod = sum % 2

const IMPOSSIBLE = "IMPOSSIBLE"

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

console.log(div)