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

Example Input: Sentence: 'a man with glasses adjusting his tie in a mirror'. Find frequency of the letter 's'
Example Output: 5

Example Input: Sentence: 'a group of cows that are standing in the grass'. Find frequency of the letter 'g'
Example Output: 3

Example Input: Sentence: 'a man stands on a tennis court with a bucket of balls'. Find frequency of the letter 'i'
Example Output:
2