import java.util.Scanner;

public class Main {
    static long max = (long) Math.pow(10, 9);

    public static void main(String[] args) {
        try (Scanner sc = new Scanner(System.in)) {
            long a = sc.nextLong();
            long b = sc.nextLong();
            for (long i = 0L; i < max; i++) {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                }
            }
            System.out.println("IMPOSSIBLE");
        }
    }
} import java.util.Scanner; Scanner java.util public class Main {
    static long max = (long) Math.pow(10, 9);

    public static void main(String[] args) {
        try (Scanner sc = new Scanner(System.in)) {
            long a = sc.nextLong();
            long b = sc.nextLong();
            for (long i = 0L; i < max; i++) {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                }
            }
            System.out.println("IMPOSSIBLE");
        }
    }
} Main static long max = (long) Math.pow(10, 9); max (long) Math.pow(10, 9) Math.pow Math 10 9 public static void main(String[] args) {
        try (Scanner sc = new Scanner(System.in)) {
            long a = sc.nextLong();
            long b = sc.nextLong();
            for (long i = 0L; i < max; i++) {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                }
            }
            System.out.println("IMPOSSIBLE");
        }
    } main {
        try (Scanner sc = new Scanner(System.in)) {
            long a = sc.nextLong();
            long b = sc.nextLong();
            for (long i = 0L; i < max; i++) {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                }
            }
            System.out.println("IMPOSSIBLE");
        }
    } try (Scanner sc = new Scanner(System.in)) {
            long a = sc.nextLong();
            long b = sc.nextLong();
            for (long i = 0L; i < max; i++) {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                }
            }
            System.out.println("IMPOSSIBLE");
        } {
            long a = sc.nextLong();
            long b = sc.nextLong();
            for (long i = 0L; i < max; i++) {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                }
            }
            System.out.println("IMPOSSIBLE");
        } Scanner sc = new Scanner(System.in) Scanner sc = new Scanner(System.in) new Scanner(System.in) long a = sc.nextLong(); a sc.nextLong() sc.nextLong sc long b = sc.nextLong(); b sc.nextLong() sc.nextLong sc for (long i = 0L; i < max; i++) {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                }
            } long i = 0L; long i = 0L; i 0L i < max i max i++ i++ i {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                }
            } {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                }
            } if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                } Math.abs(a - i) == Math.abs(b - i) Math.abs(a - i) Math.abs Math a - i a i Math.abs(b - i) Math.abs Math b - i b i {
                    System.out.println(i);
                    return;
                } System.out.println(i) System.out.println System.out System System.out i return; System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" String[] args args public class Main {
    static long max = (long) Math.pow(10, 9);

    public static void main(String[] args) {
        try (Scanner sc = new Scanner(System.in)) {
            long a = sc.nextLong();
            long b = sc.nextLong();
            for (long i = 0L; i < max; i++) {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                }
            }
            System.out.println("IMPOSSIBLE");
        }
    }
} public class Main {
    static long max = (long) Math.pow(10, 9);

    public static void main(String[] args) {
        try (Scanner sc = new Scanner(System.in)) {
            long a = sc.nextLong();
            long b = sc.nextLong();
            for (long i = 0L; i < max; i++) {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                }
            }
            System.out.println("IMPOSSIBLE");
        }
    }
} Main
import java.util.Scanner;

public class Main {
    static long max = (long) Math.pow(10, 9);

    public static void main(String[] args) {
        try (Scanner sc = new Scanner(System.in)) {
            long a = sc.nextLong();
            long b = sc.nextLong();
            for (long i = 0L; i < max; i++) {
                if (Math.abs(a - i) == Math.abs(b - i)) {
                    System.out.println(i);
                    return;
                }
            }
            System.out.println("IMPOSSIBLE");
        }
    }
}
