import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        System.out.println(((a+b)%2==0)?(a+b)/2:"IMPOSSIBLE");
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        System.out.println(((a+b)%2==0)?(a+b)/2:"IMPOSSIBLE");
    }
} Main public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        System.out.println(((a+b)%2==0)?(a+b)/2:"IMPOSSIBLE");
    } main {
        // Your code here!
        Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        System.out.println(((a+b)%2==0)?(a+b)/2:"IMPOSSIBLE");
    } 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 System.out.println(((a+b)%2==0)?(a+b)/2:"IMPOSSIBLE") System.out.println System.out System System.out ((a+b)%2==0)?(a+b)/2:"IMPOSSIBLE" ((a+b)%2==0) (a+b)%2 (a+b) a b 2 0 (a+b)/2 (a+b) a b 2 "IMPOSSIBLE" String[] args args public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        System.out.println(((a+b)%2==0)?(a+b)/2:"IMPOSSIBLE");
    }
} public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        System.out.println(((a+b)%2==0)?(a+b)/2:"IMPOSSIBLE");
    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        int b=sc.nextInt();
        System.out.println(((a+b)%2==0)?(a+b)/2:"IMPOSSIBLE");
    }
}
