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

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: 'two zebras standing around while a third zebra drinks from a pond'. Find frequency of the letter 's'
Example Output: 3

Example Input: Sentence: 'a stained glassed window shines over a clock'. Find frequency of the letter 's'
Example Output:
5