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: 'a man sitting inside a green military truck talking to someone'. Find frequency of the letter 'i'
Example Output: 7

Example Input: Sentence: 'an image of a train moving down the railroad track'. Find frequency of the letter 'a'
Example Output:
7