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

Ex Input:
Sentence: 'a group of people stand inside of a big room'. Find frequency of the letter 'o'

Ex Output:
6

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: 'a giraffe is standing in the middle of the field with its baby'. Find frequency of the letter 'm'

Ex Output:
1