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

Ex Input:
Sentence: 'a bathroom with a sink, counter top, and mirror is shown'. Find frequency of the letter 'i'

Ex Output:
4


Ex Input:
Sentence: 'a man in his car eating a donut'. Find frequency of the letter 'm'

Ex Output:
1


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

Ex Output:

5