import java.io.*; 
import java.util.*;

public class Main
{
  public static void main(String[] args) { new Main(); }
  Scanner in = new Scanner(System.in);
  PrintWriter out = new PrintWriter(System.out);
  
  int a, b;
  
  Main()
  {
    a = in.nextInt();
    b = in.nextInt();
    if ((a % 2) != (b % 2)) out.println("IMPOSSIBLE");
    else out.println((a+b)/2);
    out.close();
  }
}
/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