import java.util.Scanner;
import java.util.Arrays;
public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int A = sc.nextInt();
   int B = sc.nextInt();
   if(A>B)
   {
   if((A-B) % 2 == 1) System.out.println("IMPOSSIBLE");
    else System.out.println((A+B)/2); 
   }else{
     if((B-A) % 2 == 1) System.out.println("IMPOSSIBLE");
     else System.out.println((B+A)/2); 
   }
 }
} import java.util.Scanner; Scanner java.util import java.util.Arrays; Arrays java.util public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int A = sc.nextInt();
   int B = sc.nextInt();
   if(A>B)
   {
   if((A-B) % 2 == 1) System.out.println("IMPOSSIBLE");
    else System.out.println((A+B)/2); 
   }else{
     if((B-A) % 2 == 1) System.out.println("IMPOSSIBLE");
     else System.out.println((B+A)/2); 
   }
 }
} Main public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int A = sc.nextInt();
   int B = sc.nextInt();
   if(A>B)
   {
   if((A-B) % 2 == 1) System.out.println("IMPOSSIBLE");
    else System.out.println((A+B)/2); 
   }else{
     if((B-A) % 2 == 1) System.out.println("IMPOSSIBLE");
     else System.out.println((B+A)/2); 
   }
 } main {
   Scanner sc = new Scanner(System.in);
   int A = sc.nextInt();
   int B = sc.nextInt();
   if(A>B)
   {
   if((A-B) % 2 == 1) System.out.println("IMPOSSIBLE");
    else System.out.println((A+B)/2); 
   }else{
     if((B-A) % 2 == 1) System.out.println("IMPOSSIBLE");
     else System.out.println((B+A)/2); 
   }
 } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int A = sc.nextInt(); A sc.nextInt() sc.nextInt sc int B = sc.nextInt(); B sc.nextInt() sc.nextInt sc if(A>B)
   {
   if((A-B) % 2 == 1) System.out.println("IMPOSSIBLE");
    else System.out.println((A+B)/2); 
   }else{
     if((B-A) % 2 == 1) System.out.println("IMPOSSIBLE");
     else System.out.println((B+A)/2); 
   } A>B A B {
   if((A-B) % 2 == 1) System.out.println("IMPOSSIBLE");
    else System.out.println((A+B)/2); 
   } if((A-B) % 2 == 1) System.out.println("IMPOSSIBLE");
    else System.out.println((A+B)/2); (A-B) % 2 == 1 (A-B) % 2 (A-B) A B 2 1 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" System.out.println((A+B)/2) System.out.println System.out System System.out (A+B)/2 (A+B) A B 2 {
     if((B-A) % 2 == 1) System.out.println("IMPOSSIBLE");
     else System.out.println((B+A)/2); 
   } if((B-A) % 2 == 1) System.out.println("IMPOSSIBLE");
     else System.out.println((B+A)/2); (B-A) % 2 == 1 (B-A) % 2 (B-A) B A 2 1 System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" System.out.println((B+A)/2) System.out.println System.out System System.out (B+A)/2 (B+A) B A 2 String[] args args public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int A = sc.nextInt();
   int B = sc.nextInt();
   if(A>B)
   {
   if((A-B) % 2 == 1) System.out.println("IMPOSSIBLE");
    else System.out.println((A+B)/2); 
   }else{
     if((B-A) % 2 == 1) System.out.println("IMPOSSIBLE");
     else System.out.println((B+A)/2); 
   }
 }
} public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int A = sc.nextInt();
   int B = sc.nextInt();
   if(A>B)
   {
   if((A-B) % 2 == 1) System.out.println("IMPOSSIBLE");
    else System.out.println((A+B)/2); 
   }else{
     if((B-A) % 2 == 1) System.out.println("IMPOSSIBLE");
     else System.out.println((B+A)/2); 
   }
 }
} Main
import java.util.Scanner;
import java.util.Arrays;
public class Main{
 public static void main(String[] args){
   Scanner sc = new Scanner(System.in);
   int A = sc.nextInt();
   int B = sc.nextInt();
   if(A>B)
   {
   if((A-B) % 2 == 1) System.out.println("IMPOSSIBLE");
    else System.out.println((A+B)/2); 
   }else{
     if((B-A) % 2 == 1) System.out.println("IMPOSSIBLE");
     else System.out.println((B+A)/2); 
   }
 }
}