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

Example Input: Sentence: 'surfboard being displayed in stor window with clothes behind it'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'different sets of luggage in a baggage claim area'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'a snowboarder sits in the snow as another snowboarder stands close'. Find frequency of the letter 't'
Example Output:
4