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

Ex Input:
Sentence: 'group of snowboarders on a snowy trail overlooking a mountain'. Find frequency of the letter 'r'

Ex Output:
5


Ex Input:
Sentence: 'a red stop sign with street signs above it'. Find frequency of the letter 't'

Ex Output:
5


Ex Input:
Sentence: 'a white train car that has been painted on'. Find frequency of the letter 'd'

Ex Output:

1