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();
        calculation(a, b);
    }

    public static void calculation(int a, int b) {
        int sub = a > b ? a - b : b - a;
        if (sub % 2 == 1) {
            System.out.println("IMPOSSIBLE");
            return;
        }
        int k = sub / 2 + (a > b ? b : a);
        System.out.println(String.format("%d", k));
    }
}

/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