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 toilet of some sort near a sink of some sort'. Find frequency of the letter 'o'
Example Output: 7

Example Input: Sentence: 'a woman is riding down a hill on a child's toy'. Find frequency of the letter 's'
Example Output:
2