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

Example Input: Sentence: 'a tall building with antenna stands before a jet flying in the background'. Find frequency of the letter 't'
Example Output: 6

Example Input: Sentence: 'a person working at an airport, outside of an airplane'. Find frequency of the letter 'r'
Example Output: 5

Example Input: Sentence: 'a crowd of people standing outside of a bus'. Find frequency of the letter 'e'
Example Output:
3