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

[EX Q]: Sentence: 'a small boat and a large boat on a river near a city'. Find frequency of the letter 'a'
[EX A]: 10

[EX Q]: Sentence: 'a man skating down a stair rail in front of a bunch of people'. Find frequency of the letter 'o'
[EX A]: 5

[EX Q]: Sentence: 'some police officers on some very pretty horses by some buildings'. Find frequency of the letter 'm'
[EX A]:
3