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: 'bananas are being dried in a tropical setting'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'a white toilet sitting on a white and black checker tile floor'. Find frequency of the letter 'l'
Example Output:
4