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

Example Input: Sentence: 'a picture of two men on a motorcycle in front of a railroad track'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'bathroom scene with a black cat going behind the door'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'a man and a woman are walking down the street with an umbrella over their heads'. Find frequency of the letter 'w'
Example Output:
4