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

Example Input: Sentence: 'a teenager witting on a futon couch holding gaming controls in both hands'. Find frequency of the letter 'c'
Example Output: 3

Example Input: Sentence: 'a baseball themed birthday cake is on a serving tray'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'a lady sit under a dryer smiling with roller in her head with a smiling lady behind her'. Find frequency of the letter 'e'
Example Output:
7