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

Ex Input:
Sentence: 'three men in military suits are sitting on a bench,'. Find frequency of the letter 'a'

Ex Output:
3

Ex Input:
Sentence: 'a bunch of cows that are in the field'. Find frequency of the letter 'a'

Ex Output:
3

Ex Input:
Sentence: 'a desktop computer and mouse at a desk'. Find frequency of the letter 'o'

Ex Output:
3