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

Example Input: Sentence: 'a group of men standing around a table cutting a cake'. Find frequency of the letter 'g'
Example Output: 3

Example Input: Sentence: 'a picture of two computers on a table'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'the man is playing a game of tennis on the court'. Find frequency of the letter 'n'
Example Output:
5