import java.util.Scanner;

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)%2 != 0){
            System.out.println("IMPOSSIBLE");
            return;
        }
        
        int k = (a+b)/2;
        System.out.println(k);
    }
} import java.util.Scanner; Scanner 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)%2 != 0){
            System.out.println("IMPOSSIBLE");
            return;
        }
        
        int k = (a+b)/2;
        System.out.println(k);
    }
} Main public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        if((a-b)%2 != 0){
            System.out.println("IMPOSSIBLE");
            return;
        }
        
        int k = (a+b)/2;
        System.out.println(k);
    } main {
        
        Scanner sc = new Scanner(System.in);
        
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        if((a-b)%2 != 0){
            System.out.println("IMPOSSIBLE");
            return;
        }
        
        int k = (a+b)/2;
        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 if((a-b)%2 != 0){
            System.out.println("IMPOSSIBLE");
            return;
        } (a-b)%2 != 0 (a-b)%2 (a-b) a b 2 0 {
            System.out.println("IMPOSSIBLE");
            return;
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" return; int k = (a+b)/2; k (a+b)/2 (a+b) a b 2 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) {
        
        Scanner sc = new Scanner(System.in);
        
        int a = sc.nextInt();
        int b = sc.nextInt();
        
        if((a-b)%2 != 0){
            System.out.println("IMPOSSIBLE");
            return;
        }
        
        int k = (a+b)/2;
        System.out.println(k);
    }
} 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)%2 != 0){
            System.out.println("IMPOSSIBLE");
            return;
        }
        
        int k = (a+b)/2;
        System.out.println(k);
    }
} Main
import java.util.Scanner;

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