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

Sentence: 'a piece of film shows two street signs crossing each other'. Find frequency of the letter 'f'
2

Sentence: 'a man with a skateboard that is in the street'. Find frequency of the letter 'o'
1

Sentence: 'an airplane and busses are lined up at the airport'. Find frequency of the letter 'r'

4