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

Example Input: Sentence: 'some people sitting on surfboards in the water'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a fluffy white car looking in a mirror'. Find frequency of the letter 'k'
Example Output: 1

Example Input: Sentence: 'a dog and a cat that is laying down on a carpet'. Find frequency of the letter 't'
Example Output:
4