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

Example Input: Sentence: 'a man sitting on a motorcycle near two other men'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'man sitting on the side of a van playing the guitar'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'two women walk across a street-crossing in a city with cars and a bus driving by'. Find frequency of the letter 'c'
Example Output:
4