import java.util.Scanner;

public class Main {
    public static void main(String args[]){
        Scanner stdIn = new Scanner(System.in);
        
        int a = stdIn.nextInt();
        int b = stdIn.nextInt();
        
        int i = 0;
        for(i = 0; i <= a || i <= b; i++){
            if(Math.abs(a - i) == Math.abs(b - i)){
                System.out.println(i);
                break;
            }
        }
        
        if(i > a && i > b){
             System.out.println("IMPOSSIBLE");
        }
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String args[]){
        Scanner stdIn = new Scanner(System.in);
        
        int a = stdIn.nextInt();
        int b = stdIn.nextInt();
        
        int i = 0;
        for(i = 0; i <= a || i <= b; i++){
            if(Math.abs(a - i) == Math.abs(b - i)){
                System.out.println(i);
                break;
            }
        }
        
        if(i > a && i > b){
             System.out.println("IMPOSSIBLE");
        }
    }
} Main public static void main(String args[]){
        Scanner stdIn = new Scanner(System.in);
        
        int a = stdIn.nextInt();
        int b = stdIn.nextInt();
        
        int i = 0;
        for(i = 0; i <= a || i <= b; i++){
            if(Math.abs(a - i) == Math.abs(b - i)){
                System.out.println(i);
                break;
            }
        }
        
        if(i > a && i > b){
             System.out.println("IMPOSSIBLE");
        }
    } main {
        Scanner stdIn = new Scanner(System.in);
        
        int a = stdIn.nextInt();
        int b = stdIn.nextInt();
        
        int i = 0;
        for(i = 0; i <= a || i <= b; i++){
            if(Math.abs(a - i) == Math.abs(b - i)){
                System.out.println(i);
                break;
            }
        }
        
        if(i > a && i > b){
             System.out.println("IMPOSSIBLE");
        }
    } Scanner stdIn = new Scanner(System.in); stdIn new Scanner(System.in) int a = stdIn.nextInt(); a stdIn.nextInt() stdIn.nextInt stdIn int b = stdIn.nextInt(); b stdIn.nextInt() stdIn.nextInt stdIn int i = 0; i 0 for(i = 0; i <= a || i <= b; i++){
            if(Math.abs(a - i) == Math.abs(b - i)){
                System.out.println(i);
                break;
            }
        } i = 0; i = 0 i 0 i <= a || i <= b i <= a i a i <= b i b i++ i++ i {
            if(Math.abs(a - i) == Math.abs(b - i)){
                System.out.println(i);
                break;
            }
        } {
            if(Math.abs(a - i) == Math.abs(b - i)){
                System.out.println(i);
                break;
            }
        } if(Math.abs(a - i) == Math.abs(b - i)){
                System.out.println(i);
                break;
            } Math.abs(a - i) == Math.abs(b - i) Math.abs(a - i) Math.abs Math a - i a i Math.abs(b - i) Math.abs Math b - i b i {
                System.out.println(i);
                break;
            } System.out.println(i) System.out.println System.out System System.out i break; if(i > a && i > b){
             System.out.println("IMPOSSIBLE");
        } i > a && i > b i > a i a i > b i b {
             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 stdIn = new Scanner(System.in);
        
        int a = stdIn.nextInt();
        int b = stdIn.nextInt();
        
        int i = 0;
        for(i = 0; i <= a || i <= b; i++){
            if(Math.abs(a - i) == Math.abs(b - i)){
                System.out.println(i);
                break;
            }
        }
        
        if(i > a && i > b){
             System.out.println("IMPOSSIBLE");
        }
    }
} public class Main {
    public static void main(String args[]){
        Scanner stdIn = new Scanner(System.in);
        
        int a = stdIn.nextInt();
        int b = stdIn.nextInt();
        
        int i = 0;
        for(i = 0; i <= a || i <= b; i++){
            if(Math.abs(a - i) == Math.abs(b - i)){
                System.out.println(i);
                break;
            }
        }
        
        if(i > a && i > b){
             System.out.println("IMPOSSIBLE");
        }
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String args[]){
        Scanner stdIn = new Scanner(System.in);
        
        int a = stdIn.nextInt();
        int b = stdIn.nextInt();
        
        int i = 0;
        for(i = 0; i <= a || i <= b; i++){
            if(Math.abs(a - i) == Math.abs(b - i)){
                System.out.println(i);
                break;
            }
        }
        
        if(i > a && i > b){
             System.out.println("IMPOSSIBLE");
        }
    }
}