function main(input) {
  const args = input.split(' ');
  const A = parseInt(args[0], 10);
  const B = parseInt(args[1], 10);
  if((A+B)%2==0){
    console.log((A+B)/2);
  }else{
    console.log("IMPOSSIBLE");
  }
}

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