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

Ex Input:
Sentence: 'a man sitting inside a green military truck talking to someone'. Find frequency of the letter 'i'

Ex Output:
7

Ex Input:
Sentence: 'men are riding on motorcycles with a man riding in their carriers'. Find frequency of the letter 'r'

Ex Output:
8

Ex Input:
Sentence: 'a woman in a dress watching two people joust'. Find frequency of the letter 's'

Ex Output:
3