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

Sentence: 'a small boat travels on the water next to the shore'. Find frequency of the letter 'a'
5

Sentence: 'a number of small boats on a beach near an umbrella'. Find frequency of the letter 's'
2

Sentence: 'a stop sign with graffiti about the red sox'. Find frequency of the letter 'e'

2