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

Example Input: Sentence: 'a picture of a toilet and toilet paper in a bathroom'. Find frequency of the letter 'o'
Example Output: 5

Example Input: Sentence: 'a man sitting in front of a laptop computer next to a child'. Find frequency of the letter 'e'
Example Output: 2

Example Input: Sentence: 'a living room filled with lots of furniture and a fire place'. Find frequency of the letter 'l'
Example Output:
5