import java.util.Scanner;
public class Main {
	public static void main(String[] args) {


        // Scannerクラスのインスタンスの生成
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int k = (a + b) / 2;

        if ((a - k) == -(b - k) || -(a - k) == (b - k)) {
        	System.out.println(k);

        } 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