
import java.util.Scanner;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);

        int temp = sc.nextInt() + sc.nextInt();
        if ((int) temp / 2 == (double) temp / 2) {
            System.out.println(temp / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
        sc.close();
    }
} import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);

        int temp = sc.nextInt() + sc.nextInt();
        if ((int) temp / 2 == (double) temp / 2) {
            System.out.println(temp / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
        sc.close();
    }
} Main public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);

        int temp = sc.nextInt() + sc.nextInt();
        if ((int) temp / 2 == (double) temp / 2) {
            System.out.println(temp / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
        sc.close();
    } main {
        Scanner sc = new Scanner(System.in);

        int temp = sc.nextInt() + sc.nextInt();
        if ((int) temp / 2 == (double) temp / 2) {
            System.out.println(temp / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
        sc.close();
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int temp = sc.nextInt() + sc.nextInt(); temp sc.nextInt() + sc.nextInt() sc.nextInt() sc.nextInt sc sc.nextInt() sc.nextInt sc if ((int) temp / 2 == (double) temp / 2) {
            System.out.println(temp / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        } (int) temp / 2 == (double) temp / 2 (int) temp / 2 (int) temp 2 (double) temp / 2 (double) temp 2 {
            System.out.println(temp / 2);
        } System.out.println(temp / 2) System.out.println System.out System System.out temp / 2 temp 2 {
            System.out.println("IMPOSSIBLE");
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" sc.close() sc.close sc String[] args args public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);

        int temp = sc.nextInt() + sc.nextInt();
        if ((int) temp / 2 == (double) temp / 2) {
            System.out.println(temp / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
        sc.close();
    }
} public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);

        int temp = sc.nextInt() + sc.nextInt();
        if ((int) temp / 2 == (double) temp / 2) {
            System.out.println(temp / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
        sc.close();
    }
} Main

import java.util.Scanner;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);

        int temp = sc.nextInt() + sc.nextInt();
        if ((int) temp / 2 == (double) temp / 2) {
            System.out.println(temp / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
        sc.close();
    }
}