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

Example Input: Sentence: 'a big bear walks through some grass'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a bunch of fruit sitting in a bowl, its overflowing'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a park bench allocated as a kodak picture spot at disneyland'. Find frequency of the letter 'n'
Example Output:
3