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

Ex Input:
Sentence: 'a closet with a fan, luggage and various boxes'. Find frequency of the letter 'a'

Ex Output:
6

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: 'a laptop on top of a steel over'. Find frequency of the letter 'r'

Ex Output:
1