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

Ex Input:
Sentence: 'a man with glasses adjusting his tie in a mirror'. Find frequency of the letter 's'

Ex Output:
5

Ex Input:
Sentence: 'men are riding on motorcycles with a man riding in their carriers'. Find frequency of the letter 'r'

Ex Output:
8

Ex Input:
Sentence: 'a clock on a castle with a few clouds in the sky'. Find frequency of the letter 't'

Ex Output:
3