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

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

Sentence: 'a small wrinkley dog wearing a bow tie'. Find frequency of the letter 'e'
3

Sentence: 'a woman in a big red coat gets out of a car onto a street'. Find frequency of the letter 'o'
6