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

Ex Input:
Sentence: 'a dilapidated bench in a park under a tree'. Find frequency of the letter 'p'

Ex Output:
2

Ex Input:
Sentence: 'a man holding a frisbee high into the air'. Find frequency of the letter 'a'

Ex Output:
4

Ex Input:
Sentence: 'a young man with big muscles playing on the sand and beach'. Find frequency of the letter 'a'

Ex Output:
6