import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        final int A = scan.nextInt();
        final int B = scan.nextInt();
        scan.close();

        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) {
        Scanner scan = new Scanner(System.in);
        final int A = scan.nextInt();
        final int B = scan.nextInt();
        scan.close();

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

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

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

    public static void main(String[] args) {
        Scanner scan = new Scanner(System.in);
        final int A = scan.nextInt();
        final int B = scan.nextInt();
        scan.close();

        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) {
        Scanner scan = new Scanner(System.in);
        final int A = scan.nextInt();
        final int B = scan.nextInt();
        scan.close();

        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) {
        Scanner scan = new Scanner(System.in);
        final int A = scan.nextInt();
        final int B = scan.nextInt();
        scan.close();

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