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

Example Input: Sentence: 'a piece of film shows two street signs crossing each other'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'a black cat laying down on a bed'. Find frequency of the letter 'y'
Example Output: 1

Example Input: Sentence: 'a surfer riding on a blue surfboard on top of a small wave'. Find frequency of the letter 'l'
Example Output:
3