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

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 sheep in a field in front of other sheep'. Find frequency of the letter 'f'
Example Output: 3

Example Input: Sentence: 'a woman throwing a frisbee on a field'. Find frequency of the letter 's'
Example Output:
1

