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

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

Example Input: Sentence: 'a burning wood stove has a tea pot sitting on top of it and is full of wood'. Find frequency of the letter 'a'
Example Output: 5

Example Input: Sentence: 'a person standing next to a public transit bus'. Find frequency of the letter 'p'
Example Output:
2