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

Ex Input:
Sentence: 'a stop sign next to houses with christmas lights'. Find frequency of the letter 's'

Ex Output:
7

Ex Input:
Sentence: 'man in red shirt with black suitcase standing outside of building'. Find frequency of the letter 'i'

Ex Output:
8

Ex Input:
Sentence: 'the scale is sitting there so you can put the money in it to park there car'. Find frequency of the letter 'i'

Ex Output:
5