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

Example Input: Sentence: 'a flock of sheep grazing in a park'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'pedestrian in stripe clothing standing on street corner'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'a dog is standing on his hind legs looking out the window on a screen door'. Find frequency of the letter 'd'
Example Output:
5