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

Example Input: Sentence: 'an old boat on a beach near a large body of water'. Find frequency of the letter 'a'
Example Output: 8

Example Input: Sentence: 'a woman with short, brown hair is looking into a circular mirror and holding a camera up to her cheek'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'a person jumping into the air near a fence'. Find frequency of the letter 'a'
Example Output:
4