import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int a=scan.nextInt();
		int b=scan.nextInt();
		if((a-b)%2==0){
			if(a<b){
				System.out.println((b-a)/2+a);
			}else{
				System.out.println((a-b)/2+b);
			}
		}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