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

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: 'apples, pears, and oranges are piled up for sale'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a young baby has his hair brushed by mom'. Find frequency of the letter 'h'
Example Output:
4