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

Example Input: Sentence: 'two giraffes, one standing and one laying down'. Find frequency of the letter 'g'
Example Output: 3

Example Input: Sentence: 'a burning wood stove has a tea pot sitting on top of it and is full of wood'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a man is in a long kitchen darkly lit'. Find frequency of the letter 'e'
Example Output:
1