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

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

Example Input: Sentence: 'two boys are doing skateboard jumps in the street'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'a l shaped couch in the living room'. Find frequency of the letter 'l'
Example Output:
2