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

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 young boy holding a piece of paper on top of a table'. Find frequency of the letter 'y'
Example Output: 2

Example Input: Sentence: 'there is a white dog sitting on the ground and a cow walking by'. Find frequency of the letter 'a'
Example Output:
4