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 black bear sitting in the grass taking a rest'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a room with a king sized bed and a painting'. Find frequency of the letter 'e'
Example Output:
2