import java.util.Scanner;

public class Main {
	public static void solution(int x, int y) {
		if((x+y)%2==0) {
			System.out.println((x+y)/2);
		}
		else {
			System.out.println("IMPOSSIBLE");
		}
	}
		
	
	public static void main(String args []) {
		Scanner obj = new Scanner(System.in);
		int x = obj.nextInt();
		int y = obj.nextInt();
		solution(x,y);
	}
}
/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