import java.util.*;
class Main {
	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);
		int A = sc.nextInt();
		int B = sc.nextInt();
      if(A==0&&B==0){System.out.println("IMPOSSIBLE");}else{
      	if((A+B)%2==0){
		int K = (A+B)/2;
		if((A-K>=0&&B-K<=0)||(B-K>=0&&A-K<=0)) {
			System.out.println(K);}else{
				System.out.println("IMPOSSIBLE");
			}
        }else{
          System.out.println("IMPOSSIBLE");
        }
        }
	}
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz