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 birthday cake with a number one and three candle'. Find frequency of the letter 'w'
Example Output: 1

Example Input: Sentence: 'a group of young men standing next to each other on a  field'. Find frequency of the letter 'l'
Example Output:
1