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

Ex Input:
Sentence: 'a stuffed animal sitting on an empty bed'. Find frequency of the letter 'u'

Ex Output:
1

Ex Input:
Sentence: 'this is a sign in front of a large cathedral'. Find frequency of the letter 'a'

Ex Output:
5

Ex Input:
Sentence: 'boat sailing past a building with a large clock'. Find frequency of the letter 'a'

Ex Output:
6