
import java.util.Scanner;
public class Main{
    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        int a = Integer.parseInt(scan.next());
        int b = Integer.parseInt(scan.next());
 
        scan.close();
      
      if((a+b)%2 == 0){
        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