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

Example Input: Sentence: 'a white police vehicle parked on a paved street'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a person holding a wine glass in one hand and bread sticks in the other'. Find frequency of the letter 'd'
Example Output: 4

Example Input: Sentence: 'a bathroom with a sink, cabinet and hand mirror'. Find frequency of the letter 'i'
Example Output:
4