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

Example Input: Sentence: 'a golden haired dog looking out of an open window'. Find frequency of the letter 'd'
Example Output: 4

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 parking lot has busses some double decker'. Find frequency of the letter 'k'
Example Output:
2