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: 'two giraffes standing next to some trees'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'a person on a skateboard going down a ramp'. Find frequency of the letter 'n'
Example Output:
4