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

Example Input: Sentence: 'silhouette of person playing frisbee with moutain and sunset'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'a piece of meat covered in ketchup next to a hamburger on a green plate'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'a lady is in a store holding an umbrella upside down'. Find frequency of the letter 'a'
Example Output:
5