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

Ex Input:
Sentence: 'a train on a train track going through a bunch of trees'. Find frequency of the letter 'u'

Ex Output:
2

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: 'several cows standing on the road and grass beside it'. Find frequency of the letter 'e'

Ex Output:
5