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

Example Input: Sentence: 'a woman standing in front of a bathroom sink in front of a mirror'. Find frequency of the letter 'o'
Example Output: 8

Example Input: Sentence: 'a toilet of some sort near a sink of some sort'. Find frequency of the letter 'o'
Example Output: 7

Example Input: Sentence: 'a pastry store with  many cupcakes on display'. Find frequency of the letter 'p'
Example Output:
3