Hi kekho_thongminh, welcome to the forum. I wish you could have started with something a bit easier.
You wrote, "But can you explain the reason why the code means 17? "
As you don't know our Gus may I suggest that is one answer you can safely eliminate. :wink:
As for your post;
If we assume the dashes separate the words, we have 1,2,3,3,6,3. letter words.
Now the number of words that start with a single letter are few in number. A and I. are the only ones that come to mind.
The next word is two letters, same problem, few two letter words make a sentence if you start with ?'A'.
So let's start with ?'I' followed with (am/go).
We now have 2 three letter words. E.g. I am big. I go out. Etc.
If you get the start of the sentence, you can work out how the number matched the letter.
Well, that's how I see it, until something better comes along.