
import java.util.Scanner;

public class Main{
	public static void main(String[]args) {
		
		Scanner sc=new Scanner(System.in);
		
		int A=sc.nextInt();
		int B=sc.nextInt();
		
		if(A==B) {
			System.out.println("IMPOSSIBLE");
		}else if((A+B)%2==0){
			System.out.println((A+B)/2);
		}else {
			System.out.println("IMPOSSIBLE");
		}
		
		sc.close();
	}
}
/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