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

Ex Input:
Sentence: 'two persons standing outside a moving sub train'. Find frequency of the letter 'a'

Ex Output:
3

Ex Input:
Sentence: 'the small black and white dog holds onto a frisbee'. Find frequency of the letter 'h'

Ex Output:
3

Ex Input:
Sentence: 'two elephants walking on cement next to wall'. Find frequency of the letter 'n'

Ex Output:
5