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

Example Input: Sentence: 'a young boy bouncing a tennis ball on a tennis racquet'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'two zebras standing around while a third zebra drinks from a pond'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a woman playing a video game on the nintendo wii'. Find frequency of the letter 'o'
Example Output:
4