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

Ex Input:
Sentence: 'a small boat and a large boat on a river near a city'. Find frequency of the letter 'a'

Ex Output:
10

Ex Input:
Sentence: 'a person with some scissors over a bowl'. Find frequency of the letter 's'

Ex Output:
6

Ex Input:
Sentence: 'five snowboarders doing tricks on the snowboard course'. Find frequency of the letter 'n'

Ex Output:
4