import java.util.Scanner;

public class Main {
    static int A;
    static int B;

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

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

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

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

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

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

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

        long ans = (A + B) / 2;
        if((A + B) % 2 == 0) {
            System.out.println(ans);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} public class Main {
    static int A;
    static int B;

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

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

public class Main {
    static int A;
    static int B;

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

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