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

Ex Input:
Sentence: 'a man in a sweatshirt is taking his own picture'. Find frequency of the letter 'u'

Ex Output:
1

Ex Input:
Sentence: 'a bed with a mattress that has nothing on it'. Find frequency of the letter 'a'

Ex Output:
5

Ex Input:
Sentence: 'group of people standing around a brown and green cake on a table'. Find frequency of the letter 's'

Ex Output:
1