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

Example Input: Sentence: 'a couple of girls are hanging out at a restaurant'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'a woman in a messy kitchen with a child'. Find frequency of the letter 'a'
Example Output: 4

Example Input: Sentence: 'an interior of a living room with colored bowls lined on a counter and lights hanging from the ceiling'. Find frequency of the letter 'o'
Example Output:
10