import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int A = scanner.nextInt();
		int B = scanner.nextInt();
		scanner.close();

		if (((A + B) & 1) == 0)
			System.out.println((A + B) >> 1);
		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