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

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

Example Input: Sentence: 'a train going over a bridge with some buildings in the background'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'a refrigerator that has various stickers on it'. Find frequency of the letter 'i'
Example Output:
4