In this task, you need to count the number of times the given letter appears in the given sentence.
Example: Sentence: 'a large group of bananas proped up on several bicycles'. Find frequency of the letter 'e'
Example solution: 5
Example explanation: The letter 'e' appears 5 times in the sentence.

Problem: Sentence: 'a man sits on a living room sofa with his arms crossed'. Find frequency of the letter 'a'

Solution: 5