Can i get a straight flush?

With a standard pack of 52 cards, 5 cards are taken one by one. The first three cards are 4 of spades, 6 of spades, and 8 of spades. If the chances of getting a straight flush are stated as $\frac{a}{b}$ ,where a and b coprime. What is a + b ?

Note:

1. A straight flush is a hand that contains five cards in sequence, all of the same suit.
2. The cards in each suit rank from highest to lowest: A K Q J 10 9 8 7 6 5 4 3 2.