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

Ex Input:
Sentence: 'a black bear sitting in the grass taking a rest'. Find frequency of the letter 'a'

Ex Output:
6


Ex Input:
Sentence: 'a car accident has slowed traffic as a clean up crew works'. Find frequency of the letter 'f'

Ex Output:
2


Ex Input:
Sentence: 'a small silver u.s. air force plane sitting on a runway'. Find frequency of the letter 'l'

Ex Output:

4