import java.util.Scanner;

public class Main {

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);

    long A = Long.parseLong(sc.next());
    long B = Long.parseLong(sc.next());

    long n = Math.abs(A - (A-B) / 2);

    if (Math.abs(A-n) == Math.abs(B-n)) {
      System.out.println(n);
    } 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