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

Example Input: Sentence: 'a man serving himself pizza in a wooden booth'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a woman in yellow dress standing next to a horse'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'three vases close together have flowers in them'. Find frequency of the letter 'e'
Example Output:
9