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

Ex Input:
Sentence: 'a little child looking up to a camera holding a cell phone'. Find frequency of the letter 'g'

Ex Output:
2

Ex Input:
Sentence: 'there is a zebra that is walking enar a car'. Find frequency of the letter 'r'

Ex Output:
4

Ex Input:
Sentence: 'the female protester is displaying a relevant sign'. Find frequency of the letter 's'

Ex Output:
4