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

Example Input: Sentence: 'a bed with a mattress that has nothing on it'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a bedroom with a neatly made bed and a laptop'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'a cat relaxing on a plaid couch on a person's clothes'. Find frequency of the letter 'a'
Example Output:
6