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

Example Input: Sentence: 'a woman in a messy kitchen with a child'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'three people are gathered around sitting on a park bench'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'a park bench with a blue umbrella among flowers and trees'. Find frequency of the letter 'b'
Example Output:
3