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

[EX Q]: Sentence: 'a man sitting inside a green military truck talking to someone'. Find frequency of the letter 'i'
[EX A]: 7

[EX Q]: Sentence: 'a man opening up a package sitting on a couch'. Find frequency of the letter 'n'
[EX A]: 5

[EX Q]: Sentence: 'a train traveling down train tracks next to people'. Find frequency of the letter 's'
[EX A]:
1