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

Example Input: Sentence: 'a woman fully dressed as a geisha'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'a boy that is sitting at a table with pizza'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'a row of sparkly toilets sitting in a restroom'. Find frequency of the letter 's'
Example Output:
4