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

Example Input: Sentence: 'a couple of men standing inside of a liquor store'. Find frequency of the letter 'p'
Example Output: 1

Example Input: Sentence: 'an empty clean kitchen with island, marble countertops , stove and cabinetry'. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'a little girl using a smart phone while sitting on a chair'. Find frequency of the letter 't'
Example Output:
5