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

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 person on a skateboard going down a ramp'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a person standing on a bed wearing plain clothes and a ski board'. Find frequency of the letter 'l'
Example Output:
2