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

Example Input: Sentence: 'a display case filled with lots of different clocks'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'an airplane is being towed away from the jet bridge'. Find frequency of the letter 's'
Example Output: 1

Example Input: Sentence: 'a mirror that is in a bathroom over a sink'. Find frequency of the letter 'o'
Example Output:
4