import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);

        int first = scan.nextInt();
        int second = scan.nextInt();
        String result = "";
        int flg = (first + second) % 2;
        if (flg == 1) {
            result = "IMPOSSIBLE";
        } else {
            result = String.valueOf((first + second) / 2);
        }
        
        System.out.println(result);
    }
 }
/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