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

Example Input: Sentence: 'some food on plates next to each other on a table'. Find frequency of the letter 'e'
Example Output: 6

Example Input: Sentence: 'people plating cake in an office meeting room'. Find frequency of the letter 'g'
Example Output: 2

Example Input: Sentence: 'a cat is perched on a toilet seat to drink from the bowl'. Find frequency of the letter 'r'
Example Output:
3