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

Example Input: Sentence: 'a pizza pie sitting on a wooden table with tongs'. Find frequency of the letter 't'
Example Output: 5

Example Input: Sentence: 'girls posing of a picture one holding a tennis racket the other wearing a crown'. Find frequency of the letter 'g'
Example Output: 4

Example Input: Sentence: 'a cat peeks out of a green bag'. Find frequency of the letter 'b'
Example Output:
1