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

Example Input: Sentence: 'large grassy pasture filled with trees and a single horse'. Find frequency of the letter 'l'
Example Output: 4

Example Input: Sentence: 'a baseball player dropping his bat and starting to run'. Find frequency of the letter 'o'
Example Output: 2

Example Input: Sentence: 'red and white plane sitting in a green field'. Find frequency of the letter 's'
Example Output:
1