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: 'guy leans on truck's hood while taking pics next to parked motorcycles'. Find frequency of the letter 's'

Ex Output:
4

Ex Input:
Sentence: 'a number of people riding horses on a city street'. Find frequency of the letter 'b'

Ex Output:
1