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

Ex Input:
Sentence: 'an audience watches as a bird spreads its wings'. Find frequency of the letter 'd'

Ex Output:
3

Ex Input:
Sentence: 'a street scene looking at the back of the stop sign'. Find frequency of the letter 't'

Ex Output:
6

Ex Input:
Sentence: 'this kitchen has all white cabinets and a black microwave'. Find frequency of the letter 'e'

Ex Output:
4