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

Ex Input:
Sentence: 'cooked fish and green broccoli share a single white plate'. Find frequency of the letter 'c'

Ex Output:
3

Ex Input:
Sentence: 'two elephants facing each other touching trunks in an enclosure'. Find frequency of the letter 'f'

Ex Output:
1

Ex Input:
Sentence: 'the train has stopped on the railroad tracks'. Find frequency of the letter 'a'

Ex Output:
5