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

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: 'vanity sitting atop a base with two doors and a mirror overhead'. Find frequency of the letter 'n'
Example Output: 3

Example Input: Sentence: 'a person sitting at a table with many glasses of wine'. Find frequency of the letter 'e'
Example Output:
4