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

Ex Input:
Sentence: 'a group of people stand inside of a big room'. Find frequency of the letter 'o'

Ex Output:
6

Ex Input:
Sentence: 'a man riding a white surfboard on a large wave'. Find frequency of the letter 'a'

Ex Output:
7

Ex Input:
Sentence: 'a baseball player swinging a bat during a game'. Find frequency of the letter 'n'

Ex Output:
3