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

Example Input: Sentence: 'a man riding a white surfboard on a large wave'. Find frequency of the letter 'a'
Example Output: 7

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 surfer riding a wave in the ocean'. Find frequency of the letter 'e'
Example Output:
4