import java.util.*;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextInt();
        double a1=a/2.0;
        double b1=b/2.0;
        int sum= (int) (a1+b1);
        if ( Math.abs(a-sum) == Math.abs(b-sum)) {
            System.out.println(sum);
        }
        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