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

Sentence: 'a couple of men sitting next to each other under an umbrella'. Find frequency of the letter 'n'
5

Sentence: 'a man on stiltz is standing under a pink umbrella'. Find frequency of the letter 's'
3

Sentence: 'a surfer performs a trick over a wave'. Find frequency of the letter 'a'

4