/*
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);
      }
   } import java.util.*; util.* java import java.io.*; io.* java 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);
      }
   } 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);
      } main {
         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);
      } BufferedReader infile = new BufferedReader(new InputStreamReader(System.in)); infile new BufferedReader(new InputStreamReader(System.in)) StringTokenizer st = new StringTokenizer(infile.readLine()); st new StringTokenizer(infile.readLine()) long N = Long.parseLong(st.nextToken()); N Long.parseLong(st.nextToken()) Long.parseLong Long st.nextToken() st.nextToken st long K = Long.parseLong(st.nextToken()); K Long.parseLong(st.nextToken()) Long.parseLong Long st.nextToken() st.nextToken st if((N+K)%2 == 1)
            System.out.println("IMPOSSIBLE");
         else
            System.out.println((N+K)/2); (N+K)%2 == 1 (N+K)%2 (N+K) N K 2 1 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" System.out.println((N+K)/2) System.out.println System.out System System.out (N+K)/2 (N+K) N K 2 String args[] args 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);
      }
   } 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);
      }
   } Main
/*
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);
      }
   }