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

Example Input: Sentence: 'a number of small boats on a beach near an umbrella'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'two metal benches on an empty train platform'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a bunch of trays of hot dogs and fries on laps'. Find frequency of the letter 'f'
Example Output:
3