import java.util.*;


public class Main {
    public static void main(String[] args){

        Scanner s = new Scanner(System.in);
        
        int a = s.nextInt();
        int b = s.nextInt();

        int calc = (a + b) % 2;

        if (calc == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println((a + b) / 2);
        }
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args){

        Scanner s = new Scanner(System.in);
        
        int a = s.nextInt();
        int b = s.nextInt();

        int calc = (a + b) % 2;

        if (calc == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println((a + b) / 2);
        }
    }
} Main public static void main(String[] args){

        Scanner s = new Scanner(System.in);
        
        int a = s.nextInt();
        int b = s.nextInt();

        int calc = (a + b) % 2;

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

        Scanner s = new Scanner(System.in);
        
        int a = s.nextInt();
        int b = s.nextInt();

        int calc = (a + b) % 2;

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

        Scanner s = new Scanner(System.in);
        
        int a = s.nextInt();
        int b = s.nextInt();

        int calc = (a + b) % 2;

        if (calc == 1) {
            System.out.println("IMPOSSIBLE");
        } else {
            System.out.println((a + b) / 2);
        }
    }
} public class Main {
    public static void main(String[] args){

        Scanner s = new Scanner(System.in);
        
        int a = s.nextInt();
        int b = s.nextInt();

        int calc = (a + b) % 2;

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


public class Main {
    public static void main(String[] args){

        Scanner s = new Scanner(System.in);
        
        int a = s.nextInt();
        int b = s.nextInt();

        int calc = (a + b) % 2;

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