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

Ex Input:
Sentence: 'a red and white train passing a train station'. Find frequency of the letter 'a'

Ex Output:
7

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: 'a bunch of umbrellas that are in the sand'. Find frequency of the letter 'a'

Ex Output:
5