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

[Q]: Sentence: 'a man flying through the air on top of a skateboard'. Find frequency of the letter 'r'
[A]: 3


[Q]: Sentence: 'a picture of two men on a motorcycle in front of a railroad track'. Find frequency of the letter 'n'
[A]: 4


[Q]: Sentence: 'a police dog does a search of a rusty truck'. Find frequency of the letter 'r'
[A]:
3