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

Example Input: Sentence: 'a man flying through the air on top of a skateboard'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'a person is performing a stunt on a snowboard'. Find frequency of the letter 'p'
Example Output: 2

Example Input: Sentence: 'a baseball player holding a base on a field'. Find frequency of the letter 'b'
Example Output:
3

