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

Example Input: Sentence: 'there is a person standing on a blue towel'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a chair and umbrella on a sandy beach'. Find frequency of the letter 's'
Example Output: 1

Example Input: Sentence: 'a cat sitting on a kitchen stool yawns'. Find frequency of the letter 'c'
Example Output:
2