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

Example Input: Sentence: 'a birthday cake with a number one and three candle'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'a green bus driving in a street filled with heavy traffic'. Find frequency of the letter 'v'
Example Output: 2

Example Input: Sentence: 'two men standing on a tennis court holding tennis racquets'. Find frequency of the letter 'a'
Example Output:
3