import java.util.Scanner;
public class Main {

	public static void main(String[] args) {
		Scanner input =new Scanner(System.in);
		long a=input.nextLong();
		long b=input.nextLong();
		input.close();
		if((a+b)%2==0)
		{
			System.out.println((a+b)/2);
		}
		else
		{
			System.out.println("IMPOSSIBLE");
		}
		System.exit(0);
	

	}

}

/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