import java.io.PrintWriter;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        exec();
    }

    static void exec() {
        PrintWriter out = new PrintWriter(System.out);
        Scanner sc = new Scanner(System.in);
        int a = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());

        if((a%2==0 && b%2==0) || (a%2!=0 && b%2!=0)) {
            out.println((a+b)/2);
        }else {
            out.println("IMPOSSIBLE");
        }


        out.flush();

    }

} import java.io.PrintWriter; PrintWriter java.io import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) {
        exec();
    }

    static void exec() {
        PrintWriter out = new PrintWriter(System.out);
        Scanner sc = new Scanner(System.in);
        int a = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());

        if((a%2==0 && b%2==0) || (a%2!=0 && b%2!=0)) {
            out.println((a+b)/2);
        }else {
            out.println("IMPOSSIBLE");
        }


        out.flush();

    }

} Main public static void main(String[] args) {
        exec();
    } main {
        exec();
    } exec() exec String[] args args static void exec() {
        PrintWriter out = new PrintWriter(System.out);
        Scanner sc = new Scanner(System.in);
        int a = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());

        if((a%2==0 && b%2==0) || (a%2!=0 && b%2!=0)) {
            out.println((a+b)/2);
        }else {
            out.println("IMPOSSIBLE");
        }


        out.flush();

    } exec {
        PrintWriter out = new PrintWriter(System.out);
        Scanner sc = new Scanner(System.in);
        int a = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());

        if((a%2==0 && b%2==0) || (a%2!=0 && b%2!=0)) {
            out.println((a+b)/2);
        }else {
            out.println("IMPOSSIBLE");
        }


        out.flush();

    } PrintWriter out = new PrintWriter(System.out); out new PrintWriter(System.out) Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = Integer.parseInt(sc.next()); a Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int b = Integer.parseInt(sc.next()); b Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc if((a%2==0 && b%2==0) || (a%2!=0 && b%2!=0)) {
            out.println((a+b)/2);
        }else {
            out.println("IMPOSSIBLE");
        } (a%2==0 && b%2==0) || (a%2!=0 && b%2!=0) (a%2==0 && b%2==0) a%2==0 a%2 a 2 0 b%2==0 b%2 b 2 0 (a%2!=0 && b%2!=0) a%2!=0 a%2 a 2 0 b%2!=0 b%2 b 2 0 {
            out.println((a+b)/2);
        } out.println((a+b)/2) out.println out (a+b)/2 (a+b) a b 2 {
            out.println("IMPOSSIBLE");
        } out.println("IMPOSSIBLE") out.println out "IMPOSSIBLE" out.flush() out.flush out public class Main {

    public static void main(String[] args) {
        exec();
    }

    static void exec() {
        PrintWriter out = new PrintWriter(System.out);
        Scanner sc = new Scanner(System.in);
        int a = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());

        if((a%2==0 && b%2==0) || (a%2!=0 && b%2!=0)) {
            out.println((a+b)/2);
        }else {
            out.println("IMPOSSIBLE");
        }


        out.flush();

    }

} public class Main {

    public static void main(String[] args) {
        exec();
    }

    static void exec() {
        PrintWriter out = new PrintWriter(System.out);
        Scanner sc = new Scanner(System.in);
        int a = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());

        if((a%2==0 && b%2==0) || (a%2!=0 && b%2!=0)) {
            out.println((a+b)/2);
        }else {
            out.println("IMPOSSIBLE");
        }


        out.flush();

    }

} Main
import java.io.PrintWriter;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        exec();
    }

    static void exec() {
        PrintWriter out = new PrintWriter(System.out);
        Scanner sc = new Scanner(System.in);
        int a = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());

        if((a%2==0 && b%2==0) || (a%2!=0 && b%2!=0)) {
            out.println((a+b)/2);
        }else {
            out.println("IMPOSSIBLE");
        }


        out.flush();

    }

}
