import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if (Math.abs(a - b) % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println((a + b) / 2);
        }

    }

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

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if (Math.abs(a - b) % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println((a + b) / 2);
        }

    }

} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if (Math.abs(a - b) % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println((a + b) / 2);
        }

    } main {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if (Math.abs(a - b) % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println((a + b) / 2);
        }

    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc int b = sc.nextInt(); b sc.nextInt() sc.nextInt sc if (Math.abs(a - b) % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println((a + b) / 2);
        } Math.abs(a - b) % 2 == 1 Math.abs(a - b) % 2 Math.abs(a - b) Math.abs Math a - b a b 2 1 {
            System.out.println("IMPOSSIBLE");
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
            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 String[] args args public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if (Math.abs(a - b) % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println((a + b) / 2);
        }

    }

} public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if (Math.abs(a - b) % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println((a + b) / 2);
        }

    }

} Main
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        if (Math.abs(a - b) % 2 == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println((a + b) / 2);
        }

    }

}