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

[Q]: Sentence: 'a guy surfs a wave with his surfboard'. Find frequency of the letter 's'
[A]: 4

[Q]: Sentence: 'an area that has surrounded by a metal fence'. Find frequency of the letter 'o'
[A]: 1

[Q]: Sentence: 'a group of young men with frisbees standing next to barn'. Find frequency of the letter 'r'
[A]:
3