import java.util.*;
 
public class Main {
	public static void main (String[] args) {
		Scanner in = new Scanner(System.in);		
		int N = in.nextInt();
        int M = in.nextInt();
      
      if((M-N)%2==0){System.out.print(M-(M-N)/2);}
      else{System.out.print("IMPOSSIBLE");}
        
        
      }
	} import java.util.*; util.* java public class Main {
	public static void main (String[] args) {
		Scanner in = new Scanner(System.in);		
		int N = in.nextInt();
        int M = in.nextInt();
      
      if((M-N)%2==0){System.out.print(M-(M-N)/2);}
      else{System.out.print("IMPOSSIBLE");}
        
        
      }
	} Main public static void main (String[] args) {
		Scanner in = new Scanner(System.in);		
		int N = in.nextInt();
        int M = in.nextInt();
      
      if((M-N)%2==0){System.out.print(M-(M-N)/2);}
      else{System.out.print("IMPOSSIBLE");}
        
        
      } main {
		Scanner in = new Scanner(System.in);		
		int N = in.nextInt();
        int M = in.nextInt();
      
      if((M-N)%2==0){System.out.print(M-(M-N)/2);}
      else{System.out.print("IMPOSSIBLE");}
        
        
      } Scanner in = new Scanner(System.in); in new Scanner(System.in) int N = in.nextInt(); N in.nextInt() in.nextInt in int M = in.nextInt(); M in.nextInt() in.nextInt in if((M-N)%2==0){System.out.print(M-(M-N)/2);}
      else{System.out.print("IMPOSSIBLE");} (M-N)%2==0 (M-N)%2 (M-N) M N 2 0 {System.out.print(M-(M-N)/2);} System.out.print(M-(M-N)/2) System.out.print System.out System System.out M-(M-N)/2 M (M-N)/2 (M-N) M N 2 {System.out.print("IMPOSSIBLE");} System.out.print("IMPOSSIBLE") System.out.print System.out System System.out "IMPOSSIBLE" String[] args args public class Main {
	public static void main (String[] args) {
		Scanner in = new Scanner(System.in);		
		int N = in.nextInt();
        int M = in.nextInt();
      
      if((M-N)%2==0){System.out.print(M-(M-N)/2);}
      else{System.out.print("IMPOSSIBLE");}
        
        
      }
	} public class Main {
	public static void main (String[] args) {
		Scanner in = new Scanner(System.in);		
		int N = in.nextInt();
        int M = in.nextInt();
      
      if((M-N)%2==0){System.out.print(M-(M-N)/2);}
      else{System.out.print("IMPOSSIBLE");}
        
        
      }
	} Main
import java.util.*;
 
public class Main {
	public static void main (String[] args) {
		Scanner in = new Scanner(System.in);		
		int N = in.nextInt();
        int M = in.nextInt();
      
      if((M-N)%2==0){System.out.print(M-(M-N)/2);}
      else{System.out.print("IMPOSSIBLE");}
        
        
      }
	}