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

[Q]: Sentence: 'a person riding a skate board on a city street'. Find frequency of the letter 'a'
[A]: 5


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


[Q]: Sentence: 'a herd of black and white cows are on the grass'. Find frequency of the letter 'e'
[A]:
4