function Main(input) {
  input = input.split(' ')
  var A = parseInt(input[0])
  var B = parseInt(input[1])
  var K = (A + B) / 2

  if (Number.isInteger(K)) {
    console.log(K)
  } else {
    console.log('IMPOSSIBLE')
  }
}

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