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: 'this marketplace has a lot of fresh fruit and vegetables'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'a crowd of people stand around the plane'. Find frequency of the letter 'u'
Example Output:
1