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

Example Input: Sentence: 'a stop sign near a rail road crossing sign'. Find frequency of the letter 'g'
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: 'a young man with big muscles playing on the sand and beach'. Find frequency of the letter 'a'
Example Output:
6