import java.io.*;

/**
 * Created by Ayushi on 27/07/2019.
 */

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
        String[] temp = r.readLine().split(" ");
        long a = Long.parseLong(temp[0]);
        long b = Long.parseLong(temp[1]);
        r.close();

        if ((a+b)%2 == 1) {
            System.out.println("IMPOSSIBLE");
        }
        else {
            System.out.println((a+b) / 2);
        }
    }
} import java.io.*; io.* java public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
        String[] temp = r.readLine().split(" ");
        long a = Long.parseLong(temp[0]);
        long b = Long.parseLong(temp[1]);
        r.close();

        if ((a+b)%2 == 1) {
            System.out.println("IMPOSSIBLE");
        }
        else {
            System.out.println((a+b) / 2);
        }
    }
} Main public static void main(String[] args) throws IOException {
        BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
        String[] temp = r.readLine().split(" ");
        long a = Long.parseLong(temp[0]);
        long b = Long.parseLong(temp[1]);
        r.close();

        if ((a+b)%2 == 1) {
            System.out.println("IMPOSSIBLE");
        }
        else {
            System.out.println((a+b) / 2);
        }
    } main {
        BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
        String[] temp = r.readLine().split(" ");
        long a = Long.parseLong(temp[0]);
        long b = Long.parseLong(temp[1]);
        r.close();

        if ((a+b)%2 == 1) {
            System.out.println("IMPOSSIBLE");
        }
        else {
            System.out.println((a+b) / 2);
        }
    } BufferedReader r = new BufferedReader(new InputStreamReader(System.in)); r new BufferedReader(new InputStreamReader(System.in)) String[] temp = r.readLine().split(" "); temp r.readLine().split(" ") r.readLine().split r.readLine() r.readLine r " " long a = Long.parseLong(temp[0]); a Long.parseLong(temp[0]) Long.parseLong Long temp[0] temp 0 long b = Long.parseLong(temp[1]); b Long.parseLong(temp[1]) Long.parseLong Long temp[1] temp 1 r.close() r.close r if ((a+b)%2 == 1) {
            System.out.println("IMPOSSIBLE");
        }
        else {
            System.out.println((a+b) / 2);
        } (a+b)%2 == 1 (a+b)%2 (a+b) a b 2 1 {
            System.out.println("IMPOSSIBLE");
        } System.out.println("IMPOSSIBLE") System.out.println System.out System System.out "IMPOSSIBLE" {
            System.out.println((a+b) / 2);
        } System.out.println((a+b) / 2) System.out.println System.out System System.out (a+b) / 2 (a+b) a b 2 String[] args args public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
        String[] temp = r.readLine().split(" ");
        long a = Long.parseLong(temp[0]);
        long b = Long.parseLong(temp[1]);
        r.close();

        if ((a+b)%2 == 1) {
            System.out.println("IMPOSSIBLE");
        }
        else {
            System.out.println((a+b) / 2);
        }
    }
} public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
        String[] temp = r.readLine().split(" ");
        long a = Long.parseLong(temp[0]);
        long b = Long.parseLong(temp[1]);
        r.close();

        if ((a+b)%2 == 1) {
            System.out.println("IMPOSSIBLE");
        }
        else {
            System.out.println((a+b) / 2);
        }
    }
} Main
import java.io.*;

/**
 * Created by Ayushi on 27/07/2019.
 */

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
        String[] temp = r.readLine().split(" ");
        long a = Long.parseLong(temp[0]);
        long b = Long.parseLong(temp[1]);
        r.close();

        if ((a+b)%2 == 1) {
            System.out.println("IMPOSSIBLE");
        }
        else {
            System.out.println((a+b) / 2);
        }
    }
}
