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: 'a small boat travels on the water next to the shore'. Find frequency of the letter 'a'

Ex Output:
5

Ex Input:
Sentence: 'a man and a women who are sitting on a bench together'. Find frequency of the letter 'o'

Ex Output:
4