import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        new Main().run();
    }

    private void run() {
        Scanner scanner = new Scanner(System.in);
        int A = scanner.nextInt();
        int B = scanner.nextInt();


        if ((A + B) % 2 == 0) {
            System.out.println((A + B) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        new Main().run();
    }

    private void run() {
        Scanner scanner = new Scanner(System.in);
        int A = scanner.nextInt();
        int B = scanner.nextInt();


        if ((A + B) % 2 == 0) {
            System.out.println((A + B) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} Main public static void main(String[] args) {
        new Main().run();
    } main {
        new Main().run();
    } new Main().run() new Main().run new Main() String[] args args private void run() {
        Scanner scanner = new Scanner(System.in);
        int A = scanner.nextInt();
        int B = scanner.nextInt();


        if ((A + B) % 2 == 0) {
            System.out.println((A + B) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    } run {
        Scanner scanner = new Scanner(System.in);
        int A = scanner.nextInt();
        int B = scanner.nextInt();


        if ((A + B) % 2 == 0) {
            System.out.println((A + B) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    } Scanner scanner = new Scanner(System.in); scanner new Scanner(System.in) int A = scanner.nextInt(); A scanner.nextInt() scanner.nextInt scanner int B = scanner.nextInt(); B scanner.nextInt() scanner.nextInt scanner if ((A + B) % 2 == 0) {
            System.out.println((A + B) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        } (A + B) % 2 == 0 (A + B) % 2 (A + B) A B 2 0 {
            System.out.println((A + B) / 2);
        } System.out.println((A + B) / 2) System.out.println System.out System System.out (A + B) / 2 (A + B) A B 2 {
            System.out.println("IMPOSSIBLE");
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" public class Main {
    public static void main(String[] args) {
        new Main().run();
    }

    private void run() {
        Scanner scanner = new Scanner(System.in);
        int A = scanner.nextInt();
        int B = scanner.nextInt();


        if ((A + B) % 2 == 0) {
            System.out.println((A + B) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} public class Main {
    public static void main(String[] args) {
        new Main().run();
    }

    private void run() {
        Scanner scanner = new Scanner(System.in);
        int A = scanner.nextInt();
        int B = scanner.nextInt();


        if ((A + B) % 2 == 0) {
            System.out.println((A + B) / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        new Main().run();
    }

    private void run() {
        Scanner scanner = new Scanner(System.in);
        int A = scanner.nextInt();
        int B = scanner.nextInt();


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