import java.io.*;
import java.util.*;

public class Main {
    public static final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
    public static final PrintStream out = System.out;
    public static final PrintStream err = System.err;

    public static void main(String[] args) {
        int Ai = in.nextInt();
        int Bi = in.nextInt();

        int K2 = Math.abs(Ai) + Math.abs(Bi);

        if (K2 % 2 == 0) {
            out.println(K2 / 2);
        } else {
            out.println("IMPOSSIBLE");
        }
    }
} import java.io.*; io.* java import java.util.*; util.* java public class Main {
    public static final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
    public static final PrintStream out = System.out;
    public static final PrintStream err = System.err;

    public static void main(String[] args) {
        int Ai = in.nextInt();
        int Bi = in.nextInt();

        int K2 = Math.abs(Ai) + Math.abs(Bi);

        if (K2 % 2 == 0) {
            out.println(K2 / 2);
        } else {
            out.println("IMPOSSIBLE");
        }
    }
} Main public static final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in))); in new Scanner(new BufferedReader(new InputStreamReader(System.in))) public static final PrintStream out = System.out; out System.out System System.out public static final PrintStream err = System.err; err System.err System System.err public static void main(String[] args) {
        int Ai = in.nextInt();
        int Bi = in.nextInt();

        int K2 = Math.abs(Ai) + Math.abs(Bi);

        if (K2 % 2 == 0) {
            out.println(K2 / 2);
        } else {
            out.println("IMPOSSIBLE");
        }
    } main {
        int Ai = in.nextInt();
        int Bi = in.nextInt();

        int K2 = Math.abs(Ai) + Math.abs(Bi);

        if (K2 % 2 == 0) {
            out.println(K2 / 2);
        } else {
            out.println("IMPOSSIBLE");
        }
    } int Ai = in.nextInt(); Ai in.nextInt() in.nextInt in int Bi = in.nextInt(); Bi in.nextInt() in.nextInt in int K2 = Math.abs(Ai) + Math.abs(Bi); K2 Math.abs(Ai) + Math.abs(Bi) Math.abs(Ai) Math.abs Math Ai Math.abs(Bi) Math.abs Math Bi if (K2 % 2 == 0) {
            out.println(K2 / 2);
        } else {
            out.println("IMPOSSIBLE");
        } K2 % 2 == 0 K2 % 2 K2 2 0 {
            out.println(K2 / 2);
        } out.println(K2 / 2) out.println out K2 / 2 K2 2 {
            out.println("IMPOSSIBLE");
        } out.println("IMPOSSIBLE") out.println out "IMPOSSIBLE" String[] args args public class Main {
    public static final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
    public static final PrintStream out = System.out;
    public static final PrintStream err = System.err;

    public static void main(String[] args) {
        int Ai = in.nextInt();
        int Bi = in.nextInt();

        int K2 = Math.abs(Ai) + Math.abs(Bi);

        if (K2 % 2 == 0) {
            out.println(K2 / 2);
        } else {
            out.println("IMPOSSIBLE");
        }
    }
} public class Main {
    public static final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
    public static final PrintStream out = System.out;
    public static final PrintStream err = System.err;

    public static void main(String[] args) {
        int Ai = in.nextInt();
        int Bi = in.nextInt();

        int K2 = Math.abs(Ai) + Math.abs(Bi);

        if (K2 % 2 == 0) {
            out.println(K2 / 2);
        } else {
            out.println("IMPOSSIBLE");
        }
    }
} Main
import java.io.*;
import java.util.*;

public class Main {
    public static final Scanner in = new Scanner(new BufferedReader(new InputStreamReader(System.in)));
    public static final PrintStream out = System.out;
    public static final PrintStream err = System.err;

    public static void main(String[] args) {
        int Ai = in.nextInt();
        int Bi = in.nextInt();

        int K2 = Math.abs(Ai) + Math.abs(Bi);

        if (K2 % 2 == 0) {
            out.println(K2 / 2);
        } else {
            out.println("IMPOSSIBLE");
        }
    }
}
