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

Example Input: Sentence: 'a person standing operating a cell phone in a room'. Find frequency of the letter 'n'
Example Output: 6

Example Input: Sentence: 'a doll with huge eyes and black hair with blunt bangs holds a red umbrella'. Find frequency of the letter 'e'
Example Output: 5

Example Input: Sentence: 'a plate near a christmas tree has pasta and broccoli'. Find frequency of the letter 't'
Example Output:
4