//make sure to make new file!
import java.io.*;
import java.util.*;

public class Main{
   
   public static void main(String[] args)throws IOException{
      BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
      PrintWriter out = new PrintWriter(System.out);
      
      StringTokenizer st = new StringTokenizer(f.readLine());
      
      int n = Integer.parseInt(st.nextToken());
      int m = Integer.parseInt(st.nextToken());
      
      if((n+m) % 2 == 1){
         out.println("IMPOSSIBLE");
      } else {
         out.println((n+m)/2);
      }
      
      
      
      
      out.close();
   }
   
      
} import java.io.*; io.* java import java.util.*; util.* java public class Main{
   
   public static void main(String[] args)throws IOException{
      BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
      PrintWriter out = new PrintWriter(System.out);
      
      StringTokenizer st = new StringTokenizer(f.readLine());
      
      int n = Integer.parseInt(st.nextToken());
      int m = Integer.parseInt(st.nextToken());
      
      if((n+m) % 2 == 1){
         out.println("IMPOSSIBLE");
      } else {
         out.println((n+m)/2);
      }
      
      
      
      
      out.close();
   }
   
      
} Main public static void main(String[] args)throws IOException{
      BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
      PrintWriter out = new PrintWriter(System.out);
      
      StringTokenizer st = new StringTokenizer(f.readLine());
      
      int n = Integer.parseInt(st.nextToken());
      int m = Integer.parseInt(st.nextToken());
      
      if((n+m) % 2 == 1){
         out.println("IMPOSSIBLE");
      } else {
         out.println((n+m)/2);
      }
      
      
      
      
      out.close();
   } main {
      BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
      PrintWriter out = new PrintWriter(System.out);
      
      StringTokenizer st = new StringTokenizer(f.readLine());
      
      int n = Integer.parseInt(st.nextToken());
      int m = Integer.parseInt(st.nextToken());
      
      if((n+m) % 2 == 1){
         out.println("IMPOSSIBLE");
      } else {
         out.println((n+m)/2);
      }
      
      
      
      
      out.close();
   } BufferedReader f = new BufferedReader(new InputStreamReader(System.in)); f new BufferedReader(new InputStreamReader(System.in)) PrintWriter out = new PrintWriter(System.out); out new PrintWriter(System.out) StringTokenizer st = new StringTokenizer(f.readLine()); st new StringTokenizer(f.readLine()) int n = Integer.parseInt(st.nextToken()); n Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st int m = Integer.parseInt(st.nextToken()); m Integer.parseInt(st.nextToken()) Integer.parseInt Integer st.nextToken() st.nextToken st if((n+m) % 2 == 1){
         out.println("IMPOSSIBLE");
      } else {
         out.println((n+m)/2);
      } (n+m) % 2 == 1 (n+m) % 2 (n+m) n m 2 1 {
         out.println("IMPOSSIBLE");
      } out.println("IMPOSSIBLE") out.println out "IMPOSSIBLE" {
         out.println((n+m)/2);
      } out.println((n+m)/2) out.println out (n+m)/2 (n+m) n m 2 out.close() out.close out String[] args args public class Main{
   
   public static void main(String[] args)throws IOException{
      BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
      PrintWriter out = new PrintWriter(System.out);
      
      StringTokenizer st = new StringTokenizer(f.readLine());
      
      int n = Integer.parseInt(st.nextToken());
      int m = Integer.parseInt(st.nextToken());
      
      if((n+m) % 2 == 1){
         out.println("IMPOSSIBLE");
      } else {
         out.println((n+m)/2);
      }
      
      
      
      
      out.close();
   }
   
      
} public class Main{
   
   public static void main(String[] args)throws IOException{
      BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
      PrintWriter out = new PrintWriter(System.out);
      
      StringTokenizer st = new StringTokenizer(f.readLine());
      
      int n = Integer.parseInt(st.nextToken());
      int m = Integer.parseInt(st.nextToken());
      
      if((n+m) % 2 == 1){
         out.println("IMPOSSIBLE");
      } else {
         out.println((n+m)/2);
      }
      
      
      
      
      out.close();
   }
   
      
} Main
//make sure to make new file!
import java.io.*;
import java.util.*;

public class Main{
   
   public static void main(String[] args)throws IOException{
      BufferedReader f = new BufferedReader(new InputStreamReader(System.in));
      PrintWriter out = new PrintWriter(System.out);
      
      StringTokenizer st = new StringTokenizer(f.readLine());
      
      int n = Integer.parseInt(st.nextToken());
      int m = Integer.parseInt(st.nextToken());
      
      if((n+m) % 2 == 1){
         out.println("IMPOSSIBLE");
      } else {
         out.println((n+m)/2);
      }
      
      
      
      
      out.close();
   }
   
      
}