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

Example Input: Sentence: 'the meal is served and ready to be eaten'. Find frequency of the letter 'e'
Example Output: 8

Example Input: Sentence: 'a young boy standing in front of a counter with dougnuts'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'a black and white cat bats at its reflection in a mirror'. Find frequency of the letter 'n'
Example Output:
3