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

Example Input: Sentence: 'a group of workers washing and cleaning fruits and vegetables'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'some food on plates next to each other on a table'. Find frequency of the letter 'e'
Example Output: 6

Example Input: Sentence: 'a tray filled with lots of cupcakes covered in chocolate icing'. Find frequency of the letter 'i'
Example Output:
5

