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

Ex Input:
Sentence: 'young girl playing with refrigerator magnet while kneeling on floor'. Find frequency of the letter 'n'

Ex Output:
6

Ex Input:
Sentence: 'a boy that is sitting at a table with pizza'. Find frequency of the letter 's'

Ex Output:
2

Ex Input:
Sentence: 'a man stands in front of a herd of sheep'. Find frequency of the letter 'a'

Ex Output:
4