import java.util.Scanner;

public class Main {

   public static void main(String[] args) {
       Scanner in= new Scanner(System.in);
        int a=in.nextInt(),b=in.nextInt();
        if(a%2==b%2){
            System.out.println((a+b)/2);
        }else {
            System.out.println("IMPOSSIBLE");
        }


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

   public static void main(String[] args) {
       Scanner in= new Scanner(System.in);
        int a=in.nextInt(),b=in.nextInt();
        if(a%2==b%2){
            System.out.println((a+b)/2);
        }else {
            System.out.println("IMPOSSIBLE");
        }


       }
} Main public static void main(String[] args) {
       Scanner in= new Scanner(System.in);
        int a=in.nextInt(),b=in.nextInt();
        if(a%2==b%2){
            System.out.println((a+b)/2);
        }else {
            System.out.println("IMPOSSIBLE");
        }


       } main {
       Scanner in= new Scanner(System.in);
        int a=in.nextInt(),b=in.nextInt();
        if(a%2==b%2){
            System.out.println((a+b)/2);
        }else {
            System.out.println("IMPOSSIBLE");
        }


       } Scanner in= new Scanner(System.in); in new Scanner(System.in) int a=in.nextInt() a in.nextInt() in.nextInt in b=in.nextInt(); b in.nextInt() in.nextInt in if(a%2==b%2){
            System.out.println((a+b)/2);
        }else {
            System.out.println("IMPOSSIBLE");
        } a%2==b%2 a%2 a 2 b%2 b 2 {
            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 {
            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 in= new Scanner(System.in);
        int a=in.nextInt(),b=in.nextInt();
        if(a%2==b%2){
            System.out.println((a+b)/2);
        }else {
            System.out.println("IMPOSSIBLE");
        }


       }
} public class Main {

   public static void main(String[] args) {
       Scanner in= new Scanner(System.in);
        int a=in.nextInt(),b=in.nextInt();
        if(a%2==b%2){
            System.out.println((a+b)/2);
        }else {
            System.out.println("IMPOSSIBLE");
        }


       }
} Main
import java.util.Scanner;

public class Main {

   public static void main(String[] args) {
       Scanner in= new Scanner(System.in);
        int a=in.nextInt(),b=in.nextInt();
        if(a%2==b%2){
            System.out.println((a+b)/2);
        }else {
            System.out.println("IMPOSSIBLE");
        }


       }
}