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

Example Input: Sentence: 'a white and blue plate topped with meat and a salad'. Find frequency of the letter 'u'
Example Output: 1

Example Input: Sentence: 'a gray colored pigeon that is perched on a railing outside of a building, with other buildings in the background'. Find frequency of the letter 'e'
Example Output: 7

Example Input: Sentence: 'three people and a black car are dressed in ski jackets and two are wearing skis and our out in the snow while the third person is off to the side with her snowboard'. Find frequency of the letter 'c'
Example Output:
3