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: 'black and white photograph of man and woman sitting on a bench'. Find frequency of the letter 'w'
Example Output: 2

Example Input: Sentence: 'a man that is on a surfboard in the ocean'. Find frequency of the letter 'd'
Example Output:
1