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

Example Input: Sentence: 'a ready to eat pizza on a rack'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'a couple of boys in a field playing baseball'. Find frequency of the letter 'i'
Example Output: 3

Example Input: Sentence: 'two people are walking on the street with an umbrella'. Find frequency of the letter 'w'
Example Output:
3