def octal_to_string(octal):
    result = ""
    value_letters = [(4,"r"),(2,"w"),(1,"x")]

    for i in [int(n) for n in str(octal)]:

        for value, letter in value_letters:
            
            if i >= value:
                result += letter
                i -= value

            else:

                result += "-"

    return result
    
print(octal_to_string(755)) # Should be rwxr-xr-x
print(octal_to_string(644)) # Should be rw-r--r--
print(octal_to_string(750)) # Should be rwxr-x---
print(octal_to_string(600)) # Should be rw-------