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

Example Input: Sentence: 'a young man riding a skateboard near a forest'. Find frequency of the letter 'd'
Example Output: 2

Example Input: Sentence: 'a small boat and a large boat on a river near a city'. Find frequency of the letter 'a'
Example Output: 10

Example Input: Sentence: 'two beds in a bedroom covered with blankets next to a window'. Find frequency of the letter 'd'
Example Output:
4