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 dilapidated bench in a park under a tree'. Find frequency of the letter 'p'

Ex Output:
2

Ex Input:
Sentence: 'a black traffic light on a white street post'. Find frequency of the letter 'h'

Ex Output:
2