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: 'two people are riding on canoes down a river'. Find frequency of the letter 'e'

Ex Output:
5

Ex Input:
Sentence: 'busy city street with parked cars leading up a steep hill'. Find frequency of the letter 'y'

Ex Output:
2