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

Ex Input:
Sentence: 'a person working at an airport, outside of an airplane'. Find frequency of the letter 'r'

Ex Output:
5

Ex Input:
Sentence: 'a wedding cake with pink roses on a silver plate'. Find frequency of the letter 'a'

Ex Output:
4

Ex Input:
Sentence: 'two people on horseback riding along the beach'. Find frequency of the letter 'h'

Ex Output:
3