import java.util.Scanner;

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

        int a, b, k;

        a = sc.nextInt();
        b = sc.nextInt();

        if (a > b) {
            int temp = b;
            b = a;
            a = temp;
        }
        for (k = a; k <= b; k++) {
            if (Math.abs(a - k) == Math.abs(b - k)) {
                System.out.println(k);
                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);

        int a, b, k;

        a = sc.nextInt();
        b = sc.nextInt();

        if (a > b) {
            int temp = b;
            b = a;
            a = temp;
        }
        for (k = a; k <= b; k++) {
            if (Math.abs(a - k) == Math.abs(b - k)) {
                System.out.println(k);
                return;
            }
        }
        System.out.println("IMPOSSIBLE");
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int a, b, k;

        a = sc.nextInt();
        b = sc.nextInt();

        if (a > b) {
            int temp = b;
            b = a;
            a = temp;
        }
        for (k = a; k <= b; k++) {
            if (Math.abs(a - k) == Math.abs(b - k)) {
                System.out.println(k);
                return;
            }
        }
        System.out.println("IMPOSSIBLE");
    } main {
        Scanner sc = new Scanner(System.in);

        int a, b, k;

        a = sc.nextInt();
        b = sc.nextInt();

        if (a > b) {
            int temp = b;
            b = a;
            a = temp;
        }
        for (k = a; k <= b; k++) {
            if (Math.abs(a - k) == Math.abs(b - k)) {
                System.out.println(k);
                return;
            }
        }
        System.out.println("IMPOSSIBLE");
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a a b b k; k a = sc.nextInt() a sc.nextInt() sc.nextInt sc b = sc.nextInt() b sc.nextInt() sc.nextInt sc if (a > b) {
            int temp = b;
            b = a;
            a = temp;
        } a > b a b {
            int temp = b;
            b = a;
            a = temp;
        } int temp = b; temp b b = a b a a = temp a temp for (k = a; k <= b; k++) {
            if (Math.abs(a - k) == Math.abs(b - k)) {
                System.out.println(k);
                return;
            }
        } k = a; k = a k a k <= b k b k++ k++ k {
            if (Math.abs(a - k) == Math.abs(b - k)) {
                System.out.println(k);
                return;
            }
        } {
            if (Math.abs(a - k) == Math.abs(b - k)) {
                System.out.println(k);
                return;
            }
        } if (Math.abs(a - k) == Math.abs(b - k)) {
                System.out.println(k);
                return;
            } Math.abs(a - k) == Math.abs(b - k) Math.abs(a - k) Math.abs Math a - k a k Math.abs(b - k) Math.abs Math b - k b k {
                System.out.println(k);
                return;
            } System.out.println(k) System.out.println System.out System System.out k 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);

        int a, b, k;

        a = sc.nextInt();
        b = sc.nextInt();

        if (a > b) {
            int temp = b;
            b = a;
            a = temp;
        }
        for (k = a; k <= b; k++) {
            if (Math.abs(a - k) == Math.abs(b - k)) {
                System.out.println(k);
                return;
            }
        }
        System.out.println("IMPOSSIBLE");
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int a, b, k;

        a = sc.nextInt();
        b = sc.nextInt();

        if (a > b) {
            int temp = b;
            b = a;
            a = temp;
        }
        for (k = a; k <= b; k++) {
            if (Math.abs(a - k) == Math.abs(b - k)) {
                System.out.println(k);
                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);

        int a, b, k;

        a = sc.nextInt();
        b = sc.nextInt();

        if (a > b) {
            int temp = b;
            b = a;
            a = temp;
        }
        for (k = a; k <= b; k++) {
            if (Math.abs(a - k) == Math.abs(b - k)) {
                System.out.println(k);
                return;
            }
        }
        System.out.println("IMPOSSIBLE");
    }
}
