How many possibilities for a numerical PIN with no more than 9 digits?

Question

I understand that there would be 1,000,000,000 possibilities for a PIN of exactly nine digits (10^9, ten being the vocabulary (0-9) and the power of nine being the length of the PIN), but what about for a pin of 9 or less digits?

I am a computer security student.

Thanks.

Answer 1

you want $10+100+1000+\dots + 10^9=1111111110$