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

Example Input: Sentence: 'a person riding a skate board on a city street'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a woman holds a wine glass upside down while wearing rabbit ears and staring at a bottle of wine'. Find frequency of the letter 'a'
Example Output: 11

Example Input: Sentence: 'three red double-decker buses parked in front of a building'. Find frequency of the letter 'e'
Example Output:
8