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: 'this is a sign in front of a large cathedral'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a young girl flying a kite in a green field'. Find frequency of the letter 'r'
Example Output:
2