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

Example Input: Sentence: 'a plane with smoke coming from the tips of both wings'. Find frequency of the letter 'c'
Example Output: 1

Example Input: Sentence: 'a passenger train that is going over a bridge'. Find frequency of the letter 't'
Example Output: 3

Example Input: Sentence: 'a desktop computer on a desk next to a laptop'. Find frequency of the letter 'p'
Example Output:
4