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

Example Input: Sentence: 'a chair and umbrella on a sandy beach'. Find frequency of the letter 's'
Example Output: 1

Example Input: Sentence: 'two little girls playing with a bat and a tennis ball'. Find frequency of the letter 'w'
Example Output: 2

Example Input: Sentence: 'a pretty young lady using a smart phone'. Find frequency of the letter 'y'
Example Output:
3