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

Example Input: Sentence: 'man is snow gear and skis skiing in the snow'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'black and white photograph of man and woman sitting on a bench'. Find frequency of the letter 'w'
Example Output: 2

Example Input: Sentence: 'the three laptop computers are sitting together on the table'. Find frequency of the letter 'a'
Example Output:
3