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

Example Input: Sentence: 'children with scissors and construction paper in a school classroom'. Find frequency of the letter 'c'
Example Output: 6

Example Input: Sentence: 'a sailboat is floating near the rocky shore of a beach'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a bed sitting on a hard wood floor'. Find frequency of the letter 'n'
Example Output:
2