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

Example Input: Sentence: 'a large motorcycle that has a man dressed in old clothing next to it'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a person riding a surfboard in the middle of a wave'. Find frequency of the letter 'o'
Example Output: 3

Example Input: Sentence: 'a man hiking up a hill with skis in his hands'. Find frequency of the letter 'n'
Example Output:
4