In this task, you need to count the number of times the given letter appears in the given sentence.
--------
Question: Sentence: 'a number of small boats on a beach near an umbrella'. Find frequency of the letter 's'

Answer: 2

Question: Sentence: 'many white birds flying above a river that flows in front of a forest'. Find frequency of the letter 'a'

Answer: 5

Question: Sentence: 'a desk with two monitors and a keyboard'. Find frequency of the letter 'd'

Answer:
3