import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner stdin = new Scanner(System.in);
        int a = stdin.nextInt();
        int b = stdin.nextInt();
        if (a > b) {
            int temp = a;
            a = b;
            b = temp;
        }

        int c = Math.abs(a - b);
        if (c % 2 == 0) {
            System.out.println(b - c / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }

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

    public static void main(String[] args) {
        Scanner stdin = new Scanner(System.in);
        int a = stdin.nextInt();
        int b = stdin.nextInt();
        if (a > b) {
            int temp = a;
            a = b;
            b = temp;
        }

        int c = Math.abs(a - b);
        if (c % 2 == 0) {
            System.out.println(b - c / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }

} Main public static void main(String[] args) {
        Scanner stdin = new Scanner(System.in);
        int a = stdin.nextInt();
        int b = stdin.nextInt();
        if (a > b) {
            int temp = a;
            a = b;
            b = temp;
        }

        int c = Math.abs(a - b);
        if (c % 2 == 0) {
            System.out.println(b - c / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    } main {
        Scanner stdin = new Scanner(System.in);
        int a = stdin.nextInt();
        int b = stdin.nextInt();
        if (a > b) {
            int temp = a;
            a = b;
            b = temp;
        }

        int c = Math.abs(a - b);
        if (c % 2 == 0) {
            System.out.println(b - c / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    } Scanner stdin = new Scanner(System.in); stdin new Scanner(System.in) int a = stdin.nextInt(); a stdin.nextInt() stdin.nextInt stdin int b = stdin.nextInt(); b stdin.nextInt() stdin.nextInt stdin if (a > b) {
            int temp = a;
            a = b;
            b = temp;
        } a > b a b {
            int temp = a;
            a = b;
            b = temp;
        } int temp = a; temp a a = b a b b = temp b temp int c = Math.abs(a - b); c Math.abs(a - b) Math.abs Math a - b a b if (c % 2 == 0) {
            System.out.println(b - c / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        } c % 2 == 0 c % 2 c 2 0 {
            System.out.println(b - c / 2);
        } System.out.println(b - c / 2) System.out.println System.out System System.out b - c / 2 b c / 2 c 2 {
            System.out.println("IMPOSSIBLE");
        } 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 stdin = new Scanner(System.in);
        int a = stdin.nextInt();
        int b = stdin.nextInt();
        if (a > b) {
            int temp = a;
            a = b;
            b = temp;
        }

        int c = Math.abs(a - b);
        if (c % 2 == 0) {
            System.out.println(b - c / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }

} public class Main {

    public static void main(String[] args) {
        Scanner stdin = new Scanner(System.in);
        int a = stdin.nextInt();
        int b = stdin.nextInt();
        if (a > b) {
            int temp = a;
            a = b;
            b = temp;
        }

        int c = Math.abs(a - b);
        if (c % 2 == 0) {
            System.out.println(b - c / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }

} Main
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner stdin = new Scanner(System.in);
        int a = stdin.nextInt();
        int b = stdin.nextInt();
        if (a > b) {
            int temp = a;
            a = b;
            b = temp;
        }

        int c = Math.abs(a - b);
        if (c % 2 == 0) {
            System.out.println(b - c / 2);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }

}
