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: 'some cubbies with towels in a blue bathroom'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'the back of a train with graffiti on it'. Find frequency of the letter 'k'
Example Output:
1