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

Ex Input:
Sentence: 'a cat that is standing next to an open suitcase'. Find frequency of the letter 'e'

Ex Output:
3

Ex Input:
Sentence: 'apples, pears, and oranges are piled up for sale'. Find frequency of the letter 'a'

Ex Output:
6

Ex Input:
Sentence: 'a silver sedan is parked in a plaza lot on a cloudy day'. Find frequency of the letter 'o'

Ex Output:
3