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

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: 'an old boat on a beach near a large body of water'. Find frequency of the letter 'a'
Example Output: 8

Example Input: Sentence: 'a man sitting at a table looking at a laptop'. Find frequency of the letter 'a'
Example Output:
8