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

Example Input: Sentence: 'a group of three dogs laying on a kitchen floor'. Find frequency of the letter 'r'
Example Output: 3

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

Example Input: Sentence: 'a child hitches a ride on dad's wheeled luggage at the airport'. Find frequency of the letter 's'
Example Output:
2