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

Example Input: Sentence: 'a photo of a toilet which is not clean'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'a birthday cake with a number one and three candle'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'a prson riding a skate board down a hill'. Find frequency of the letter 'g'
Example Output:
1