In this task, you need to count the number of times the given letter appears in the given sentence.

Ex Input:
Sentence: 'a person dressed all in black snowboarding down a slope'. Find frequency of the letter 'e'

Ex Output:
4

Ex Input:
Sentence: 'a couple of men sitting next to each other under an umbrella'. Find frequency of the letter 'n'

Ex Output:
5

Ex Input:
Sentence: 'a professional tennis player holding a tennis racket at the us open'. Find frequency of the letter 'o'

Ex Output:
4