import java.util.*;

public class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);

        int A = sc.nextInt();
        int B = sc.nextInt();

        if((A-B)/2==(double)(A-B)/2){
            if(A>=B){
                System.out.println(A-(A-B)/2);
            }
            if(A<B){
                System.out.println(B-(B-A)/2);
            }
        }
        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