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 giraffes walk together by a hill'. Find frequency of the letter 'b'

Ex Output:
1

Ex Input:
Sentence: 'a jet that is flying in the sky with a banner behind it'. Find frequency of the letter 'r'

Ex Output:
1

Ex Input:
Sentence: 'a group of people standing together in the snow, some holding skis'. Find frequency of the letter 'e'

Ex Output:
6