a) What is the number of sequences of length 20 of 1, -1 so that the total sum of the numbers is 0?

The answer is 20 choose 10 right?

This is what I'm stuck on:

b) How many of the sequences in part a) have a positive sum for the first k numbers and a negative sum for the first m numbers? (where 1≤k≤m≤20)

Thanks