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: 'food sitting in bowls on table with checked tablecloth'. Find frequency of the letter 't'
Example Output: 6

Example Input: Sentence: 'a plate with french fries and a hot dog and beef sandwich topped with cheese'. Find frequency of the letter 'n'
Example Output:
4