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

Example Input: Sentence: 'small airplane in the process of taking off'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a young elephant is touching a bigger elephant with its trunk'. Find frequency of the letter 'i'
Example Output: 5

Example Input: Sentence: 'a street sign posed on the side of a street by a building'. Find frequency of the letter 's'
Example Output:
5