import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextInt();
        long b = sc.nextInt();
        if (a % 2 == 0 && b % 2 == 0
                || a % 2 == 1 && b % 2 == 1) {
            System.out.println((a + b) / 2);
            return;
        }
        System.out.println("IMPOSSIBLE");
    }

} import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextInt();
        long b = sc.nextInt();
        if (a % 2 == 0 && b % 2 == 0
                || a % 2 == 1 && b % 2 == 1) {
            System.out.println((a + b) / 2);
            return;
        }
        System.out.println("IMPOSSIBLE");
    }

} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextInt();
        long b = sc.nextInt();
        if (a % 2 == 0 && b % 2 == 0
                || a % 2 == 1 && b % 2 == 1) {
            System.out.println((a + b) / 2);
            return;
        }
        System.out.println("IMPOSSIBLE");
    } main {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextInt();
        long b = sc.nextInt();
        if (a % 2 == 0 && b % 2 == 0
                || a % 2 == 1 && b % 2 == 1) {
            System.out.println((a + b) / 2);
            return;
        }
        System.out.println("IMPOSSIBLE");
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long a = sc.nextInt(); a sc.nextInt() sc.nextInt sc long b = sc.nextInt(); b sc.nextInt() sc.nextInt sc if (a % 2 == 0 && b % 2 == 0
                || a % 2 == 1 && b % 2 == 1) {
            System.out.println((a + b) / 2);
            return;
        } a % 2 == 0 && b % 2 == 0
                || a % 2 == 1 && b % 2 == 1 a % 2 == 0 && b % 2 == 0 a % 2 == 0 a % 2 a 2 0 b % 2 == 0 b % 2 b 2 0 a % 2 == 1 && b % 2 == 1 a % 2 == 1 a % 2 a 2 1 b % 2 == 1 b % 2 b 2 1 {
            System.out.println((a + b) / 2);
            return;
        } System.out.println((a + b) / 2) System.out.println System.out System System.out (a + b) / 2 (a + b) a b 2 return; 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 sc = new Scanner(System.in);
        long a = sc.nextInt();
        long b = sc.nextInt();
        if (a % 2 == 0 && b % 2 == 0
                || a % 2 == 1 && b % 2 == 1) {
            System.out.println((a + b) / 2);
            return;
        }
        System.out.println("IMPOSSIBLE");
    }

} public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextInt();
        long b = sc.nextInt();
        if (a % 2 == 0 && b % 2 == 0
                || a % 2 == 1 && b % 2 == 1) {
            System.out.println((a + b) / 2);
            return;
        }
        System.out.println("IMPOSSIBLE");
    }

} Main
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextInt();
        long b = sc.nextInt();
        if (a % 2 == 0 && b % 2 == 0
                || a % 2 == 1 && b % 2 == 1) {
            System.out.println((a + b) / 2);
            return;
        }
        System.out.println("IMPOSSIBLE");
    }

}
