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

Example Input: Sentence: 'a ready to eat pizza on a rack'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'a man using a video game controller in a room'. Find frequency of the letter 'n'
Example Output: 4

Example Input: Sentence: 'a plaque on the side of a stone bench'. Find frequency of the letter 'd'
Example Output:
1