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

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 stuffed animal sitting on an empty bed'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a stuffed animal sitting on top of a bench'. Find frequency of the letter 'a'
Example Output:
4