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

Example Input: Sentence: 'a surfer wipes out on a huge wave'. Find frequency of the letter 'a'
Example Output: 3

Example Input: Sentence: 'a white toilet sitting next to a bathroom sink'. Find frequency of the letter 'b'
Example Output: 1

Example Input: Sentence: 'someone is holing a camera over a plate and food on it'. Find frequency of the letter 'a'
Example Output:
6