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

Example Input: Sentence: 'pedestrian in stripe clothing standing on street corner'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'a crowd of fans watch soccer players from stadium seating'. Find frequency of the letter 'f'
Example Output: 3

Example Input: Sentence: 'a large family spread out and seated around a living room'. Find frequency of the letter 'd'
Example Output:
4