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

Example Input: Sentence: 'small airplane in the process of taking off'. Find frequency of the letter 'e'
Example Output: 3

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: 'two sheep standing on top of a grass covered hill'. Find frequency of the letter 'i'
Example Output:
2