import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long A = Integer.parseInt(sc.next());
        long B = Integer.parseInt(sc.next());
        double K = (A + B) / 2d;

        if (K == (long) K) {
            System.out.println((long) K);
        } else {
            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);
        long A = Integer.parseInt(sc.next());
        long B = Integer.parseInt(sc.next());
        double K = (A + B) / 2d;

        if (K == (long) K) {
            System.out.println((long) K);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long A = Integer.parseInt(sc.next());
        long B = Integer.parseInt(sc.next());
        double K = (A + B) / 2d;

        if (K == (long) K) {
            System.out.println((long) K);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    } main {
        Scanner sc = new Scanner(System.in);
        long A = Integer.parseInt(sc.next());
        long B = Integer.parseInt(sc.next());
        double K = (A + B) / 2d;

        if (K == (long) K) {
            System.out.println((long) K);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long A = Integer.parseInt(sc.next()); A Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc long B = Integer.parseInt(sc.next()); B Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc double K = (A + B) / 2d; K (A + B) / 2d (A + B) A B 2d if (K == (long) K) {
            System.out.println((long) K);
        } else {
            System.out.println("IMPOSSIBLE");
        } K == (long) K K (long) K {
            System.out.println((long) K);
        } System.out.println((long) K) System.out.println System.out System System.out (long) K {
            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 sc = new Scanner(System.in);
        long A = Integer.parseInt(sc.next());
        long B = Integer.parseInt(sc.next());
        double K = (A + B) / 2d;

        if (K == (long) K) {
            System.out.println((long) K);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long A = Integer.parseInt(sc.next());
        long B = Integer.parseInt(sc.next());
        double K = (A + B) / 2d;

        if (K == (long) K) {
            System.out.println((long) K);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
} Main
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        long A = Integer.parseInt(sc.next());
        long B = Integer.parseInt(sc.next());
        double K = (A + B) / 2d;

        if (K == (long) K) {
            System.out.println((long) K);
        } else {
            System.out.println("IMPOSSIBLE");
        }
    }
}