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

[Q]: Sentence: 'a man that is on his cell phone'. Find frequency of the letter 's'
[A]: 2


[Q]: Sentence: 'a group of people stand inside of a big room'. Find frequency of the letter 'o'
[A]: 6


[Q]: Sentence: 'a group of jockeys riding horses around a track'. Find frequency of the letter 'r'
[A]:
5