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

Example Input: Sentence: 'a white toilet sitting next to a toilet paper roller'. Find frequency of the letter 'o'
Example Output: 4

Example Input: Sentence: 'a red train traveling down train tracks near a forest'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a boy in green shirt sitting on chair by microwave'. Find frequency of the letter 'r'
Example Output:
4