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

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 couple of boys in a field playing baseball'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'a giraffe investigates a tree stump in a wildlife enclosure'. Find frequency of the letter 's'
Example Output:
4