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

[EX Q]: Sentence: 'a man in a sweatshirt is taking his own picture'. Find frequency of the letter 'u'
[EX A]: 1

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

[EX Q]: Sentence: 'street sign at an intersection with buildings in the background'. Find frequency of the letter 'r'
[EX A]:
3