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

Example Input: Sentence: 'a double decker transit bus parks by a blue on the side of the street in front of a brick building in the city'. Find frequency of the letter 't'
Example Output: 9

Example Input: Sentence: 'a man opening up a package sitting on a couch'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a boy on a skateboard walking a dog on a pavement'. Find frequency of the letter 'g'
Example Output:
2