import java.util.Scanner;

public class Main {

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

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

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        long a = scanner.nextInt();
        long b = scanner.nextInt();
        long k = (a + b) / 2;
        if (k * 2 == a + b) System.out.println(k);
        else System.out.println("IMPOSSIBLE");
    }
}
