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

Example Input: Sentence: 'a small white plane sitting on top of the ocean'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'a group of people standing around a table filled with assorted electronic devices'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'vegetables are displayed inside of a lunch box of some kind'. Find frequency of the letter 'i'
Example Output:
4