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

[Q]: Sentence: 'a doorway looking into a small bathroom with white toilet'. Find frequency of the letter 'r'
[A]: 2


[Q]: Sentence: 'a police officer who is riding on a horse'. Find frequency of the letter 'p'
[A]: 1


[Q]: Sentence: 'the batter prepares to swing at the next pitch as the catcher and umpire look on'. Find frequency of the letter 'n'
[A]:
4