import java.util.*;


public class Main {
    public static void main(String[] args) {
      
      Scanner sc = new Scanner(System.in);
      int a = Integer.parseInt(sc.next());
      int b = Integer.parseInt(sc.next());
      if ((a +b) % 2 == 0) {
        System.out.print((a +b) / 2);
      } else {
        System.out.print("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