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 makes some cock tales at a desk'. Find frequency of the letter 'k'
3

Sentence: 'several cows are laying down in a field'. Find frequency of the letter 'd'

2