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

Ex Input:
Sentence: 'an airplane is being towed away from the jet bridge'. Find frequency of the letter 's'

Ex Output:
1


Ex Input:
Sentence: 'two zebras standing around while a third zebra drinks from a pond'. Find frequency of the letter 's'

Ex Output:
3


Ex Input:
Sentence: 'a sign on a electrical post that is about juice'. Find frequency of the letter 's'

Ex Output:

3