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

[Q]: Sentence: 'a man serving himself pizza in a wooden booth'. Find frequency of the letter 'n'
[A]: 4

[Q]: Sentence: 'a fluffy white car looking in a mirror'. Find frequency of the letter 'k'
[A]: 1

[Q]: Sentence: 'two young girls relaxing on their luggage at the airport'. Find frequency of the letter 'a'
[A]:
4