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

Example Input: Sentence: 'a couple of items that are on the wall'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'two men are playing a video game as another sits watching'. Find frequency of the letter 'a'
Example Output: 7

Example Input: Sentence: 'a tennis player after hitting a shot back to the other side of the court'. Find frequency of the letter 'n'
Example Output:
3