import java.util.*;
public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextLong();
        sc.close();
        if((a + b) % 2 == 0){
            System.out.println((a + b) / 2);
        }else{
            System.out.println("IMPOSSIBLE");
        }
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextLong();
        sc.close();
        if((a + b) % 2 == 0){
            System.out.println((a + b) / 2);
        }else{
            System.out.println("IMPOSSIBLE");
        }
    }
} Main public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextLong();
        sc.close();
        if((a + b) % 2 == 0){
            System.out.println((a + b) / 2);
        }else{
            System.out.println("IMPOSSIBLE");
        }
    } main {
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextLong();
        sc.close();
        if((a + b) % 2 == 0){
            System.out.println((a + b) / 2);
        }else{
            System.out.println("IMPOSSIBLE");
        }
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) long a = sc.nextLong(); a sc.nextLong() sc.nextLong sc long b = sc.nextLong(); b sc.nextLong() sc.nextLong sc sc.close() sc.close sc if((a + b) % 2 == 0){
            System.out.println((a + b) / 2);
        }else{
            System.out.println("IMPOSSIBLE");
        } (a + b) % 2 == 0 (a + b) % 2 (a + b) a b 2 0 {
            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 sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextLong();
        sc.close();
        if((a + b) % 2 == 0){
            System.out.println((a + b) / 2);
        }else{
            System.out.println("IMPOSSIBLE");
        }
    }
} public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextLong();
        sc.close();
        if((a + b) % 2 == 0){
            System.out.println((a + b) / 2);
        }else{
            System.out.println("IMPOSSIBLE");
        }
    }
} Main
import java.util.*;
public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        long a = sc.nextLong();
        long b = sc.nextLong();
        sc.close();
        if((a + b) % 2 == 0){
            System.out.println((a + b) / 2);
        }else{
            System.out.println("IMPOSSIBLE");
        }
    }
}