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

Example Input: Sentence: 'there is a woman pointing at something with her dog'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'a  woman standing next to a man on a motorcycle in front of a market'. Find frequency of the letter 'l'
Example Output: 1

Example Input: Sentence: 'a couple of police cars are parked outside a building'. Find frequency of the letter 'a'
Example Output:
5