
import java.util.*;






// Warning: Printing unwanted or ill-formatted data to output will cause the test cases to fail

public class Main{
    public static void main(String args[] ) throws Exception {
        
        //Scanner
        Scanner scanner = new Scanner(System.in);
        long a=0,b=0;
        a = scanner.nextLong();
        b = scanner.nextLong();
        long sum = a+b;
        if(sum%2==0) {
            System.out.println(sum/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        }
        
    }
    
    
    
} import java.util.*; util.* java public class Main{
    public static void main(String args[] ) throws Exception {
        
        //Scanner
        Scanner scanner = new Scanner(System.in);
        long a=0,b=0;
        a = scanner.nextLong();
        b = scanner.nextLong();
        long sum = a+b;
        if(sum%2==0) {
            System.out.println(sum/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        }
        
    }
    
    
    
} Main public static void main(String args[] ) throws Exception {
        
        //Scanner
        Scanner scanner = new Scanner(System.in);
        long a=0,b=0;
        a = scanner.nextLong();
        b = scanner.nextLong();
        long sum = a+b;
        if(sum%2==0) {
            System.out.println(sum/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        }
        
    } main {
        
        //Scanner
        Scanner scanner = new Scanner(System.in);
        long a=0,b=0;
        a = scanner.nextLong();
        b = scanner.nextLong();
        long sum = a+b;
        if(sum%2==0) {
            System.out.println(sum/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        }
        
    } Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) long a=0 a 0 b=0; b 0 a = scanner.nextLong() a scanner.nextLong() scanner.nextLong scanner b = scanner.nextLong() b scanner.nextLong() scanner.nextLong scanner long sum = a+b; sum a+b a b if(sum%2==0) {
            System.out.println(sum/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        } sum%2==0 sum%2 sum 2 0 {
            System.out.println(sum/2);
        } System.out.println(sum/2) System.out.println System.out System System.out sum/2 sum 2 {
            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[] ) throws Exception {
        
        //Scanner
        Scanner scanner = new Scanner(System.in);
        long a=0,b=0;
        a = scanner.nextLong();
        b = scanner.nextLong();
        long sum = a+b;
        if(sum%2==0) {
            System.out.println(sum/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        }
        
    }
    
    
    
} public class Main{
    public static void main(String args[] ) throws Exception {
        
        //Scanner
        Scanner scanner = new Scanner(System.in);
        long a=0,b=0;
        a = scanner.nextLong();
        b = scanner.nextLong();
        long sum = a+b;
        if(sum%2==0) {
            System.out.println(sum/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        }
        
    }
    
    
    
} Main

import java.util.*;






// Warning: Printing unwanted or ill-formatted data to output will cause the test cases to fail

public class Main{
    public static void main(String args[] ) throws Exception {
        
        //Scanner
        Scanner scanner = new Scanner(System.in);
        long a=0,b=0;
        a = scanner.nextLong();
        b = scanner.nextLong();
        long sum = a+b;
        if(sum%2==0) {
            System.out.println(sum/2);
        }
        else {
            System.out.println("IMPOSSIBLE");
        }
        
    }
    
    
    
}
