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

Example Input: Sentence: 'a man standing on a tennis court holding a racquet'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'some cubbies with towels in a blue bathroom'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'two giraffes standing by their fence looking over it'. Find frequency of the letter 'a'
Example Output:
2