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

Example Input: Sentence: 'a white toilet sitting next to a toilet paper roller'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'a girl hitting a tennis ball on a court'. Find frequency of the letter 'l'
Example Output: 3

Example Input: Sentence: 'a person looking at their cell phone with a christmas tree on it'. Find frequency of the letter 'l'
Example Output:
3