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

Ex Input:
Sentence: 'a close up of a train on a track in the dark'. Find frequency of the letter 'a'

Ex Output:
6

Ex Input:
Sentence: 'an image of a cautionary construction sign'. Find frequency of the letter 'a'

Ex Output:
5

Ex Input:
Sentence: 'large black bear walking alongside a dirt road alone'. Find frequency of the letter 'l'

Ex Output:
5