import java.io.*;

/**
 * Created by Ayushi on 27/07/2019.
 */

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
        String[] temp = r.readLine().split(" ");
        long a = Long.parseLong(temp[0]);
        long b = Long.parseLong(temp[1]);
        r.close();

        if ((a+b)%2 == 1) {
            System.out.println("IMPOSSIBLE");
        }
        else {
            System.out.println((a+b) / 2);
        }
    }
}

/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