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

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 person with some scissors over a bowl'. Find frequency of the letter 's'
Example Output: 6

Example Input: Sentence: 'a man taking a picture of a man on a motorcycle'. Find frequency of the letter 'a'
Example Output:
7