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

Example Input: Sentence: 'a woman cross country skiing in a competition'. Find frequency of the letter 's'
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: 'a view of a person's legs sitting on a bench alone'. Find frequency of the letter 'o'
Example Output:
4