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

Example Input: Sentence: 'a parking meter painted green with a sign that describes it as not a parking meter'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'flower in a vase full of water sitting on a stand '. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'a kitchen has stainless steel appliances and wooden cabinets'. Find frequency of the letter 'n'
Example Output:
6