
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        long A = scanner.nextLong();
        long B = scanner.nextLong();

        if ((A + B) % 2 != 0) {
            System.out.println("IMPOSSIBLE");
            return;
        }

        System.out.println((A + B) / 2);
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        long A = scanner.nextLong();
        long B = scanner.nextLong();

        if ((A + B) % 2 != 0) {
            System.out.println("IMPOSSIBLE");
            return;
        }

        System.out.println((A + B) / 2);
    }
} Main public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        long A = scanner.nextLong();
        long B = scanner.nextLong();

        if ((A + B) % 2 != 0) {
            System.out.println("IMPOSSIBLE");
            return;
        }

        System.out.println((A + B) / 2);
    } main {
        Scanner scanner = new Scanner(System.in);

        long A = scanner.nextLong();
        long B = scanner.nextLong();

        if ((A + B) % 2 != 0) {
            System.out.println("IMPOSSIBLE");
            return;
        }

        System.out.println((A + B) / 2);
    } Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) long A = scanner.nextLong(); A scanner.nextLong() scanner.nextLong scanner long B = scanner.nextLong(); B scanner.nextLong() scanner.nextLong scanner 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; System.out.println((A + B) / 2) System.out.println System.out System System.out (A + B) / 2 (A + B) A B 2 String[] args args public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        long A = scanner.nextLong();
        long B = scanner.nextLong();

        if ((A + B) % 2 != 0) {
            System.out.println("IMPOSSIBLE");
            return;
        }

        System.out.println((A + B) / 2);
    }
} public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        long A = scanner.nextLong();
        long B = scanner.nextLong();

        if ((A + B) % 2 != 0) {
            System.out.println("IMPOSSIBLE");
            return;
        }

        System.out.println((A + B) / 2);
    }
} Main

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        long A = scanner.nextLong();
        long B = scanner.nextLong();

        if ((A + B) % 2 != 0) {
            System.out.println("IMPOSSIBLE");
            return;
        }

        System.out.println((A + B) / 2);
    }
}
