import java.util.*;
import java.util.InputMismatchException;
public class Main {
	public static void main(String[] args){
      try{
		Scanner sc = new Scanner(System.in);
     
	      int a = sc.nextInt();
	      int b = sc.nextInt();
	      int k;
      
	      if(a == b){
 	  	     System.out.println("IMPOSSIBLE");
          }else if(a < 0){
            System.out.println("IMPOSSIBLE");
          }else if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } 
      }catch(InputMismatchException e){
        System.out.println("IMPOSSIBLE");
      }
	}
} import java.util.*; util.* java import java.util.InputMismatchException; InputMismatchException java.util public class Main {
	public static void main(String[] args){
      try{
		Scanner sc = new Scanner(System.in);
     
	      int a = sc.nextInt();
	      int b = sc.nextInt();
	      int k;
      
	      if(a == b){
 	  	     System.out.println("IMPOSSIBLE");
          }else if(a < 0){
            System.out.println("IMPOSSIBLE");
          }else if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } 
      }catch(InputMismatchException e){
        System.out.println("IMPOSSIBLE");
      }
	}
} Main public static void main(String[] args){
      try{
		Scanner sc = new Scanner(System.in);
     
	      int a = sc.nextInt();
	      int b = sc.nextInt();
	      int k;
      
	      if(a == b){
 	  	     System.out.println("IMPOSSIBLE");
          }else if(a < 0){
            System.out.println("IMPOSSIBLE");
          }else if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } 
      }catch(InputMismatchException e){
        System.out.println("IMPOSSIBLE");
      }
	} main {
      try{
		Scanner sc = new Scanner(System.in);
     
	      int a = sc.nextInt();
	      int b = sc.nextInt();
	      int k;
      
	      if(a == b){
 	  	     System.out.println("IMPOSSIBLE");
          }else if(a < 0){
            System.out.println("IMPOSSIBLE");
          }else if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } 
      }catch(InputMismatchException e){
        System.out.println("IMPOSSIBLE");
      }
	} try{
		Scanner sc = new Scanner(System.in);
     
	      int a = sc.nextInt();
	      int b = sc.nextInt();
	      int k;
      
	      if(a == b){
 	  	     System.out.println("IMPOSSIBLE");
          }else if(a < 0){
            System.out.println("IMPOSSIBLE");
          }else if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } 
      }catch(InputMismatchException e){
        System.out.println("IMPOSSIBLE");
      } catch(InputMismatchException e){
        System.out.println("IMPOSSIBLE");
      } InputMismatchException e {
        System.out.println("IMPOSSIBLE");
      } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
		Scanner sc = new Scanner(System.in);
     
	      int a = sc.nextInt();
	      int b = sc.nextInt();
	      int k;
      
	      if(a == b){
 	  	     System.out.println("IMPOSSIBLE");
          }else if(a < 0){
            System.out.println("IMPOSSIBLE");
          }else if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } 
      } 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 int k; k if(a == b){
 	  	     System.out.println("IMPOSSIBLE");
          }else if(a < 0){
            System.out.println("IMPOSSIBLE");
          }else if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } a == b a b {
 	  	     System.out.println("IMPOSSIBLE");
          } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" if(a < 0){
            System.out.println("IMPOSSIBLE");
          }else if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } a < 0 a 0 {
            System.out.println("IMPOSSIBLE");
          } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } b > 1000000000 b 1000000000 {
            System.out.println("IMPOSSIBLE");
	      } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } k=(b+a)/2 k (b+a)/2 (b+a) b a 2 if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            } (k*2) != (a+b) (k*2) k 2 (a+b) a b {
              System.out.println("IMPOSSIBLE");
            } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
			System.out.println(k);
            } System.out.println(k) System.out.println System.out System System.out k String[] args args public class Main {
	public static void main(String[] args){
      try{
		Scanner sc = new Scanner(System.in);
     
	      int a = sc.nextInt();
	      int b = sc.nextInt();
	      int k;
      
	      if(a == b){
 	  	     System.out.println("IMPOSSIBLE");
          }else if(a < 0){
            System.out.println("IMPOSSIBLE");
          }else if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } 
      }catch(InputMismatchException e){
        System.out.println("IMPOSSIBLE");
      }
	}
} public class Main {
	public static void main(String[] args){
      try{
		Scanner sc = new Scanner(System.in);
     
	      int a = sc.nextInt();
	      int b = sc.nextInt();
	      int k;
      
	      if(a == b){
 	  	     System.out.println("IMPOSSIBLE");
          }else if(a < 0){
            System.out.println("IMPOSSIBLE");
          }else if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } 
      }catch(InputMismatchException e){
        System.out.println("IMPOSSIBLE");
      }
	}
} Main
import java.util.*;
import java.util.InputMismatchException;
public class Main {
	public static void main(String[] args){
      try{
		Scanner sc = new Scanner(System.in);
     
	      int a = sc.nextInt();
	      int b = sc.nextInt();
	      int k;
      
	      if(a == b){
 	  	     System.out.println("IMPOSSIBLE");
          }else if(a < 0){
            System.out.println("IMPOSSIBLE");
          }else if (b > 1000000000){
            System.out.println("IMPOSSIBLE");
	      }else{
	        k=(b+a)/2;
            if((k*2) != (a+b)){
              System.out.println("IMPOSSIBLE");
            }else{
			System.out.println(k);
            }
	      } 
      }catch(InputMismatchException e){
        System.out.println("IMPOSSIBLE");
      }
	}
}
