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

Ex Input:
Sentence: 'a number of small boats on a beach near an umbrella'. Find frequency of the letter 's'

Ex Output:
2

Ex Input:
Sentence: 'there are two men sitting on a couch playing video games'. Find frequency of the letter 'v'

Ex Output:
1

Ex Input:
Sentence: 'a plane sitting behind a fence on private property'. Find frequency of the letter 'b'

Ex Output:
1