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

Ex Input:
Sentence: 'a black computer with a screen reading "press any key to begin."'. Find frequency of the letter 'a'

Ex Output:
5


Ex Input:
Sentence: 'a person is water skiing and there are birds flying around above them'. Find frequency of the letter 'n'

Ex Output:
5


Ex Input:
Sentence: 'a dessert with white cream is on a white plate'. Find frequency of the letter 'i'

Ex Output:

4