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: 'man riding a skate board down the little board way'. Find frequency of the letter 'k'
Example Output: 1

Example Input: Sentence: 'a traffic signal showing that it is safe for horses to cross'. Find frequency of the letter 's'
Example Output:
8