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

Example Input: Sentence: 'this marketplace has a lot of fresh fruit and vegetables'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'a man opening up a package sitting on a couch'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a street sign giving directions is posted outside'. Find frequency of the letter 'e'
Example Output:
5