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

Example Input: Sentence: 'this yellow fire hydrant sits in a grass field'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'this is a sign in front of a large cathedral'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a female tennis player hitting a ball on a tennis court'. Find frequency of the letter 'm'
Example Output:
1