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

Example Input: Sentence: 'a couple of men sitting next to each other under an umbrella'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a man pushing a cart full of bananas'. Find frequency of the letter 'u'
Example Output: 2

Example Input: Sentence: 'some people playing with some kites on top of a hill'. Find frequency of the letter 'p'
Example Output:
4