import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int  k = 0 ;
        for(int i = 0 ; i<Integer.MAX_VALUE ; i++){
        if((a-i)==-(b-i)){
            k = i;
            break ;
        }
       
}
        if(k>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) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int  k = 0 ;
        for(int i = 0 ; i<Integer.MAX_VALUE ; i++){
        if((a-i)==-(b-i)){
            k = i;
            break ;
        }
       
}
        if(k>0) System.out.println(k);
        else System.out.println("IMPOSSIBLE");
    }
} Main public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int  k = 0 ;
        for(int i = 0 ; i<Integer.MAX_VALUE ; i++){
        if((a-i)==-(b-i)){
            k = i;
            break ;
        }
       
}
        if(k>0) System.out.println(k);
        else System.out.println("IMPOSSIBLE");
    } main {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int  k = 0 ;
        for(int i = 0 ; i<Integer.MAX_VALUE ; i++){
        if((a-i)==-(b-i)){
            k = i;
            break ;
        }
       
}
        if(k>0) System.out.println(k);
        else System.out.println("IMPOSSIBLE");
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc int b = sc.nextInt(); b sc.nextInt() sc.nextInt sc int  k = 0 ; k 0 for(int i = 0 ; i<Integer.MAX_VALUE ; i++){
        if((a-i)==-(b-i)){
            k = i;
            break ;
        }
       
} int i = 0 ; int i = 0 ; i 0 i<Integer.MAX_VALUE i Integer.MAX_VALUE Integer Integer.MAX_VALUE i++ i++ i {
        if((a-i)==-(b-i)){
            k = i;
            break ;
        }
       
} {
        if((a-i)==-(b-i)){
            k = i;
            break ;
        }
       
} if((a-i)==-(b-i)){
            k = i;
            break ;
        } (a-i)==-(b-i) (a-i) a i -(b-i) (b-i) b i {
            k = i;
            break ;
        } k = i k i break ; if(k>0) System.out.println(k);
        else System.out.println("IMPOSSIBLE"); k>0 k 0 System.out.println(k) System.out.println System.out System System.out k 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);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int  k = 0 ;
        for(int i = 0 ; i<Integer.MAX_VALUE ; i++){
        if((a-i)==-(b-i)){
            k = i;
            break ;
        }
       
}
        if(k>0) System.out.println(k);
        else System.out.println("IMPOSSIBLE");
    }
} public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int  k = 0 ;
        for(int i = 0 ; i<Integer.MAX_VALUE ; i++){
        if((a-i)==-(b-i)){
            k = i;
            break ;
        }
       
}
        if(k>0) System.out.println(k);
        else System.out.println("IMPOSSIBLE");
    }
} Main
import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int  k = 0 ;
        for(int i = 0 ; i<Integer.MAX_VALUE ; i++){
        if((a-i)==-(b-i)){
            k = i;
            break ;
        }
       
}
        if(k>0) System.out.println(k);
        else System.out.println("IMPOSSIBLE");
    }
}
