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

Example Input: Sentence: 'the child is waving a small flag as she stands near two women'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'a man in costume talking on a cell phone'. Find frequency of the letter 'm'
Example Output: 2

Example Input: Sentence: 'a kite is flying in the air over a sandy beach as a person windsurfs below'. Find frequency of the letter 'b'
Example Output:
2