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

Ex Input:
Sentence: 'a man with glasses adjusting his tie in a mirror'. Find frequency of the letter 's'

Ex Output:
5

Ex Input:
Sentence: 'a toasted sub sandwich is next to a salad'. Find frequency of the letter 'a'

Ex Output:
6

Ex Input:
Sentence: 'woman sitting on luggage cart smiles up at the camera'. Find frequency of the letter 'n'

Ex Output:
3