import java.util.Scanner;

public class Main {

    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        String[] abt = sc.nextLine().split(" ");
        int A = Integer.parseInt(abt[0]);
        int B = Integer.parseInt(abt[1]);

        boolean isMatch = false;
        for (int i = 0; i < Math.max(A, B); i++) {

            if (Math.abs(A - i) == Math.abs(B - i)) {
                System.out.println(i);
                isMatch = true;
            }
        }
        if (!isMatch) {
            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