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

Example Input: Sentence: 'a glass vase sits between two statues'. Find frequency of the letter 'o'
Example Output: 1

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: 'an in-n-out burger stall of some sort somewhere'. Find frequency of the letter 't'
Example Output:
3