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

Ex Input:
Sentence: 'a double decker bus with a lot of people on board'. Find frequency of the letter 'l'

Ex Output:
3

Ex Input:
Sentence: 'decorated baseball cake in a clear plastic box'. Find frequency of the letter 'k'

Ex Output:
1

Ex Input:
Sentence: 'the street signs say do not enter and one way'. Find frequency of the letter 'e'

Ex Output:
6