/*
Roses are red
Memes are neat
All my test cases time out
Lmao yeet
*/
import java.util.*;
import java.io.*;

   public class Main
   {
      public static void main(String args[]) throws Exception
      {
         BufferedReader infile = new BufferedReader(new InputStreamReader(System.in));  
         StringTokenizer st = new StringTokenizer(infile.readLine());
         long N = Long.parseLong(st.nextToken());
         long K = Long.parseLong(st.nextToken());
         if((N+K)%2 == 1)
            System.out.println("IMPOSSIBLE");
         else
            System.out.println((N+K)/2);
      }
   }
/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