import java.util.Scanner;

public class Main {
    public static void main(String args[]){
        Scanner stdIn = new Scanner(System.in);
        
        int a = stdIn.nextInt();
        int b = stdIn.nextInt();
        
        int i = 0;
        for(i = 0; i <= a || i <= b; i++){
            if(Math.abs(a - i) == Math.abs(b - i)){
                System.out.println(i);
                break;
            }
        }
        
        if(i > a && i > b){
             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