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

[EX Q]: Sentence: 'an old woman is holding her cat in front of a camera'. Find frequency of the letter 'r'
[EX A]: 3

[EX Q]: Sentence: 'a red train traveling down train tracks near a forest'. Find frequency of the letter 'f'
[EX A]: 1

[EX Q]: Sentence: 'a couple men riding skis down a snow covered slope'. Find frequency of the letter 'n'
[EX A]:
4