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

Example Input: Sentence: 'a young boy standing in front of a counter with dougnuts'. Find frequency of the letter 'o'
Example Output: 6

Example Input: Sentence: 'a little child looking up to a camera holding a cell phone'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'an adult and a child sleeping in a bed'. Find frequency of the letter 'a'
Example Output:
5