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

Sentence: 'a log in the water with three birds perched on it'. Find frequency of the letter 'r'
4

Sentence: 'a police officer who is riding on a horse'. Find frequency of the letter 'p'
1

Sentence: 'two young ladies are riding their horses on the beach'. Find frequency of the letter 'o'

4