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

[EX Q]: Sentence: 'a woman throwing a frisbee on a field'. Find frequency of the letter 's'
[EX A]: 1

[EX Q]: Sentence: 'a train going over a bridge with some buildings in the background'. Find frequency of the letter 'd'
[EX A]: 3

[EX Q]: Sentence: 'a yellow fire hydrant sitting at the top of a green hill'. Find frequency of the letter 'r'
[EX A]:
3