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 close up of a plate with a whole carrot'. Find frequency of the letter 'w'
Example Output: 2

Example Input: Sentence: 'an orange and black fire hydrant at the edge of a street'. Find frequency of the letter 'h'
Example Output:
2