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

Example Input: Sentence: 'a pizza pie sitting on a wooden table with tongs'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'a small boat and a large boat on a river near a city'. Find frequency of the letter 'a'
Example Output: 10

Example Input: Sentence: 'a modern building has a long table holding many fruits and vegetables'. Find frequency of the letter 'n'
Example Output:
6