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

Example Input: Sentence: 'two elephants walking on cement next to wall'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a bunch of broken down trains sit in a yard'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a man in a red shirt throws a white frisbee'. Find frequency of the letter 'a'
Example Output:
4