import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        String[] in = s.nextLine().split(" ");
        int a = Integer.parseInt(in[0]);
        int b = Integer.parseInt(in[1]);
        if((b-a) % 2 != 0){
            System.out.println("IMPOSSIBLE");
        }else{
            System.out.println((b-a) / 2 + a);
        }
    }
}
/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