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

Example Input: Sentence: 'a plastic tray that has different foods inside of it'. Find frequency of the letter 't'
Example Output: 6

Example Input: Sentence: 'a bunch of green bananas is hanging on a tree'. Find frequency of the letter 'n'
Example Output: 7

Example Input: Sentence: 'a group of people on a street next to buildings'. Find frequency of the letter 'p'
Example Output:
3