function main(input){
  input = input.split("\n");
//  const N = parseInt(input[0]);
  const AB = input[0].split(" ").map(x => parseInt(x))
//  console.log(AB);

  if((AB[0]+AB[1])%2 == 1){
    console.log('IMPOSSIBLE');
  }else{
    console.log((AB[0]+AB[1])/2);
  }


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