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: 'some giraffes moving across a big grass field'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'the bright blue background makes the street sign easy to read'. Find frequency of the letter 'y'
Example Output:
1