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: 'a man and a woman are walking down the street with an umbrella over their heads'. Find frequency of the letter 'w'

Ex Output:
4


Ex Input:
Sentence: 'a train is sitting at a train station'. Find frequency of the letter 'r'

Ex Output:

2