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

Example input: Sentence: 'a large group of bananas proped up on several bicycles'. Find frequency of the letter 'e'
Example output: 5
Example explanation: The letter 'e' appears 5 times in the sentence.
Q: Sentence: 'a lady is standing near a luggage stand at the air port'. Find frequency of the letter 'a'
A:
9