import java.util.Scanner;

public class Main {

    public static void main(String args[] ) throws Exception {


        Scanner s = new Scanner(System.in);


        int a = s.nextInt();    // Reading input from STDIN
        int b = s.nextInt();

        int sum = a+b;
        int k = sum/2;

        if(sum%2==0){
            System.out.println(k);
        }else{
            System.out.println("IMPOSSIBLE");
        }

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

    public static void main(String args[] ) throws Exception {


        Scanner s = new Scanner(System.in);


        int a = s.nextInt();    // Reading input from STDIN
        int b = s.nextInt();

        int sum = a+b;
        int k = sum/2;

        if(sum%2==0){
            System.out.println(k);
        }else{
            System.out.println("IMPOSSIBLE");
        }

    }
} Main public static void main(String args[] ) throws Exception {


        Scanner s = new Scanner(System.in);


        int a = s.nextInt();    // Reading input from STDIN
        int b = s.nextInt();

        int sum = a+b;
        int k = sum/2;

        if(sum%2==0){
            System.out.println(k);
        }else{
            System.out.println("IMPOSSIBLE");
        }

    } main {


        Scanner s = new Scanner(System.in);


        int a = s.nextInt();    // Reading input from STDIN
        int b = s.nextInt();

        int sum = a+b;
        int k = sum/2;

        if(sum%2==0){
            System.out.println(k);
        }else{
            System.out.println("IMPOSSIBLE");
        }

    } Scanner s = new Scanner(System.in); s new Scanner(System.in) int a = s.nextInt(); a s.nextInt() s.nextInt s int b = s.nextInt(); b s.nextInt() s.nextInt s int sum = a+b; sum a+b a b int k = sum/2; k sum/2 sum 2 if(sum%2==0){
            System.out.println(k);
        }else{
            System.out.println("IMPOSSIBLE");
        } sum%2==0 sum%2 sum 2 0 {
            System.out.println(k);
        } System.out.println(k) System.out.println System.out System System.out 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[] ) throws Exception {


        Scanner s = new Scanner(System.in);


        int a = s.nextInt();    // Reading input from STDIN
        int b = s.nextInt();

        int sum = a+b;
        int k = sum/2;

        if(sum%2==0){
            System.out.println(k);
        }else{
            System.out.println("IMPOSSIBLE");
        }

    }
} public class Main {

    public static void main(String args[] ) throws Exception {


        Scanner s = new Scanner(System.in);


        int a = s.nextInt();    // Reading input from STDIN
        int b = s.nextInt();

        int sum = a+b;
        int k = sum/2;

        if(sum%2==0){
            System.out.println(k);
        }else{
            System.out.println("IMPOSSIBLE");
        }

    }
} Main
import java.util.Scanner;

public class Main {

    public static void main(String args[] ) throws Exception {


        Scanner s = new Scanner(System.in);


        int a = s.nextInt();    // Reading input from STDIN
        int b = s.nextInt();

        int sum = a+b;
        int k = sum/2;

        if(sum%2==0){
            System.out.println(k);
        }else{
            System.out.println("IMPOSSIBLE");
        }

    }
}

