import java.util.Scanner;


public class Main{

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		int A = input.nextInt();
		int B = input.nextInt();
		if ((A + B) % 2 ==0) {
			System.out.print((A+B)/2);

		}else {
			System.out.print("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