import java.util.*;
import java.util.Arrays;
public class Main{
    
    
    public static void main(String[] args){
       Scanner sc = new Scanner(System.in);
        double a = sc.nextDouble();
        double b = sc.nextDouble();
        int k = 0;
        if((a+b)%2==0){
            k = (int)((a+b)/2);
            System.out.println(k);

        }
        else{
            System.out.println("IMPOSSIBLE");
        }
        
    }
   
    
       
    
} import java.util.*; util.* java import java.util.Arrays; Arrays java.util public class Main{
    
    
    public static void main(String[] args){
       Scanner sc = new Scanner(System.in);
        double a = sc.nextDouble();
        double b = sc.nextDouble();
        int k = 0;
        if((a+b)%2==0){
            k = (int)((a+b)/2);
            System.out.println(k);

        }
        else{
            System.out.println("IMPOSSIBLE");
        }
        
    }
   
    
       
    
} Main public static void main(String[] args){
       Scanner sc = new Scanner(System.in);
        double a = sc.nextDouble();
        double b = sc.nextDouble();
        int k = 0;
        if((a+b)%2==0){
            k = (int)((a+b)/2);
            System.out.println(k);

        }
        else{
            System.out.println("IMPOSSIBLE");
        }
        
    } main {
       Scanner sc = new Scanner(System.in);
        double a = sc.nextDouble();
        double b = sc.nextDouble();
        int k = 0;
        if((a+b)%2==0){
            k = (int)((a+b)/2);
            System.out.println(k);

        }
        else{
            System.out.println("IMPOSSIBLE");
        }
        
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) double a = sc.nextDouble(); a sc.nextDouble() sc.nextDouble sc double b = sc.nextDouble(); b sc.nextDouble() sc.nextDouble sc int k = 0; k 0 if((a+b)%2==0){
            k = (int)((a+b)/2);
            System.out.println(k);

        }
        else{
            System.out.println("IMPOSSIBLE");
        } (a+b)%2==0 (a+b)%2 (a+b) a b 2 0 {
            k = (int)((a+b)/2);
            System.out.println(k);

        } k = (int)((a+b)/2) k (int)((a+b)/2) (a+b) a b 2 System.out.println(k) System.out.println System.out System System.out k {
            System.out.println("IMPOSSIBLE");
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" String[] args args public class Main{
    
    
    public static void main(String[] args){
       Scanner sc = new Scanner(System.in);
        double a = sc.nextDouble();
        double b = sc.nextDouble();
        int k = 0;
        if((a+b)%2==0){
            k = (int)((a+b)/2);
            System.out.println(k);

        }
        else{
            System.out.println("IMPOSSIBLE");
        }
        
    }
   
    
       
    
} public class Main{
    
    
    public static void main(String[] args){
       Scanner sc = new Scanner(System.in);
        double a = sc.nextDouble();
        double b = sc.nextDouble();
        int k = 0;
        if((a+b)%2==0){
            k = (int)((a+b)/2);
            System.out.println(k);

        }
        else{
            System.out.println("IMPOSSIBLE");
        }
        
    }
   
    
       
    
} Main
import java.util.*;
import java.util.Arrays;
public class Main{
    
    
    public static void main(String[] args){
       Scanner sc = new Scanner(System.in);
        double a = sc.nextDouble();
        double b = sc.nextDouble();
        int k = 0;
        if((a+b)%2==0){
            k = (int)((a+b)/2);
            System.out.println(k);

        }
        else{
            System.out.println("IMPOSSIBLE");
        }
        
    }
   
    
       
    
}