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

Example Input: Sentence: 'a lady is standing near a luggage stand at the air port'. Find frequency of the letter 'a'
Example Output: 9

Example Input: Sentence: 'a desk covered in notepads, a laptop and other devices'. Find frequency of the letter 'c'
Example Output: 2

Example Input: Sentence: 'a father and son play a video game on the nintendo wii'. Find frequency of the letter 'o'
Example Output:
4