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

Example Input: Sentence: 'a dog has found a rather large stick for playing fetch'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'an area that has surrounded by a metal fence'. Find frequency of the letter 'o'
Example Output: 1

Example Input: Sentence: 'a man plays wii while two other men sit on the couch and watch'. Find frequency of the letter 'h'
Example Output:
5