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

Ex Input:
Sentence: 'a large bathroom with marble floor, counter, and bath'. Find frequency of the letter 'f'

Ex Output:
1

Ex Input:
Sentence: 'a chair and umbrella on a sandy beach'. Find frequency of the letter 's'

Ex Output:
1

Ex Input:
Sentence: 'an commercial airline airplane flies above the clouds'. Find frequency of the letter 'n'

Ex Output:
3