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

Example Input: Sentence: 'two giraffes standing next to some trees'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'a group of tennis players on a tennis court'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'a young boy wearing black and holding a skateboard on the sidewalk'. Find frequency of the letter 'n'
Example Output:
5