import java.util.Scanner;

public class Main {

   public static void main(String[] args) {
       Scanner in= new Scanner(System.in);
        int a=in.nextInt(),b=in.nextInt();
        if(a%2==b%2){
            System.out.println((a+b)/2);
        }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