import java.util.*;

public class Main{

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

    long a1,a2=0,b1,b2=0;

    for(long i = 0;i < 1000000000;i++) {

      a1 = a-i;
      a2 = Math.abs(a1);
      b1 = b-i;
      b2 = Math.abs(b1);

      if(a2 == b2) {
        System.out.println(i);
        break;
      }

    }

    if(a2 != b2) {
      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