import java.util.*;

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

        int tmp = (a + b) / 2;
        if(Math.abs(a - tmp) == Math.abs(b - tmp)) {
            System.out.println(tmp);
        } else if(Math.abs(a - tmp + 1) == Math.abs(b - tmp + 1)) {
            System.out.println(tmp - 1);
        } else if(Math.abs(a - tmp - 1) == Math.abs(b - tmp - 1)) {
            System.out.println(tmp + 1);
        } else {
            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