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

Ex Input:
Sentence: 'an area that has surrounded by a metal fence'. Find frequency of the letter 'o'

Ex Output:
1

Ex Input:
Sentence: 'a couple is cutting in to a cake at a wedding'. Find frequency of the letter 'd'

Ex Output:
2

Ex Input:
Sentence: 'a red couch in the center of a large room with a bookshelf and a coffee table'. Find frequency of the letter 'r'

Ex Output:
4