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

Ex Input:
Sentence: 'a giraffe shares a habitat with a zebra, an ostrich, and an okapi'. Find frequency of the letter 'e'

Ex Output:
3


Ex Input:
Sentence: 'a passenger bus that is driving down the street'. Find frequency of the letter 'r'

Ex Output:
3


Ex Input:
Sentence: 'a photograph of a speed limit sign displaying "20 twenty's plenty."'. Find frequency of the letter 'd'

Ex Output:

2