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

Example Input: Sentence: 'a man and a woman are walking down the street with an umbrella over their heads'. Find frequency of the letter 'w'
Example Output: 4

Example Input: Sentence: 'a group of three dogs laying on a kitchen floor'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a man is swinging a bat at a ball while the catcher crouches behind him'. Find frequency of the letter 'h'
Example Output:
6