import java.util.*;
public class Main {

	public static void main(String[] args) {
		Scanner stdIn = new Scanner(System.in);
		int A = stdIn.nextInt();
		int B = stdIn.nextInt();
		
		if((Math.max(A,B)-Math.min(A,B))%2 == 0) {
			System.out.println((Math.max(A,B)+Math.min(A,B))/2);
		}
		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