import java.util.Scanner;

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

        if (Math.abs(a - b) % 2 != 0) {
            System.out.println("IMPOSSIBLE");
        } else {
            int tmp = Math.abs(a - b) / 2;
            if (a < b) {
                System.out.println(a + tmp);
            } else {
                System.out.println(b + tmp);
            }
        }
    }
}

/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