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

Sentence: 'men standing in the grass with a frisbee'. Find frequency of the letter 'a'
3

Sentence: 'vanity sitting atop a base with two doors and a mirror overhead'. Find frequency of the letter 'n'
3

Sentence: 'a white toilet sitting on a white and black checker tile floor'. Find frequency of the letter 'l'

4