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();
		boolean isBothNumberMenus = true;
		int index = 1;
		while(isBothNumberMenus) {
			if(a - index < 0 && b - index < 0) {
				System.out.println("IMPOSSIBLE");
				break;
			}
			
			if(Math.abs(a - index) == Math.abs(b - index)) {
				System.out.println(index);
				break;
			}
			index++;
			
		}
		

		
		

		
		
	}	

}
/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