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

Example Input: Sentence: 'a guy riding a bike close to a car in the street'. Find frequency of the letter 'i'
Example Output: 4

Example Input: Sentence: 'a group of people standing on grass with a train in the background'. Find frequency of the letter 'n'
Example Output: 6

Example Input: Sentence: 'a man walking towards a group of livestock'. Find frequency of the letter 'a'
Example Output:
5