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

Example Input: Sentence: 'this is a close up view of a tennis player during the game'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'a small wrinkley dog wearing a bow tie'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'several cars are driving down a busy street'. Find frequency of the letter 'r'
Example Output:
5