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

Example Input: Sentence: 'a man skating down a stair rail in front of a bunch of people'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a woman is taking a sip of a drink from a glass'. Find frequency of the letter 'i'
Example Output: 4

Example Input: Sentence: 'a man is standing on the platform of a red double-decker bus which is in traffic on the street.'. Find frequency of the letter 'e'
Example Output:
8