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 on a skateboard going down a ramp'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a bathroom with two mirrors, sink and shower stall'. Find frequency of the letter 'o'
Example Output:
5