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

Example Input: Sentence: 'bathroom scene with a black cat going behind the door'. Find frequency of the letter 'e'
Example Output: 4

Example Input: Sentence: 'a photo of a small boat on a shore'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'a large building with a clock affixed to the top of it'. Find frequency of the letter 'd'
Example Output:
2