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

Example Input: Sentence: 'a green bus driving in a street filled with heavy traffic'. Find frequency of the letter 'v'
Example Output: 2

Example Input: Sentence: 'a woman cross country skiing in a competition'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a person that is holding an umbrella outside'. Find frequency of the letter 'a'
Example Output:
4