import java.util.Scanner;

public class Main {

    public static void main(String args[] ) throws Exception {


        Scanner s = new Scanner(System.in);


        int a = s.nextInt();    // Reading input from STDIN
        int b = s.nextInt();

        int sum = a+b;
        int k = sum/2;

        if(sum%2==0){
            System.out.println(k);
        }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