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

Example Input: Sentence: 'a man riding his skateboard down low as he turns'. Find frequency of the letter 'g'
Example Output: 1

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

Example Input: Sentence: 'an abandoned car and a cow on a field'. Find frequency of the letter 'c'
Example Output:
2