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

Ex Input:
Sentence: 'two computers and a laptop set up on a desk'. Find frequency of the letter 'r'

Ex Output:
1

Ex Input:
Sentence: 'party cakes in plastic containers, with a cake with a plastic santa claus and reindeer on top'. Find frequency of the letter 's'

Ex Output:
6

Ex Input:
Sentence: 'a man speaking to a woman holding a tea cup'. Find frequency of the letter 'k'

Ex Output:
1