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

Example Input: Sentence: 'two boys are doing skateboard jumps in the street'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'a man standing on a tennis court holding a racquet'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'clock on pedestal in front of older building'. Find frequency of the letter 'a'
Example Output:
1