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

Example Input: Sentence: 'two giraffes, one standing and one laying down'. Find frequency of the letter 'g'
Example Output: 3

Example Input: Sentence: 'a snow filled mountain with a guy with ski's'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a boy petting a frog on a frisbee'. Find frequency of the letter 'n'
Example Output:
2