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: 'a large group of motorcycles parked along a street'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'an old black and white photo of two people'. Find frequency of the letter 'o'
Example Output:
6