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

Example Input: Sentence: 'an old woman is holding her cat in front of a camera'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'a woman fully dressed as a geisha'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'bicycles sit solidarity on a train car against the window'. Find frequency of the letter 't'
Example Output:
5