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

Example Input: Sentence: 'young boy flying kite in large field at outdoor event'. Find frequency of the letter 'y'
Example Output: 3

Example Input: Sentence: 'a man riding on the back of a white horse next to other horses'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'a couple of horses standing near a road'. Find frequency of the letter 'h'
Example Output:
1