function main(arg){
  arg = arg.trim().split(' ').map(str => parseInt(str, 10));
  var a = arg[0], b = arg[1];
  var out = ((a+b)%2 === 0) ? (a+b)/2 : 'IMPOSSIBLE';
  console.log(out);
}
main(require('fs').readFileSync('/dev/stdin','utf8'));