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

Example Input: Sentence: 'a person is riding in a cart pulled by a horse'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'a person does a trick on a ramp on a snowboard'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a couple of young boys riding on a brown and a black horse'. Find frequency of the letter 'w'
Example Output:
1