import java.util.Scanner;

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

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