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 man flying through the air on top of a skateboard'. Find frequency of the letter 'r'

Ex Output:
3

Ex Input:
Sentence: 'a giraffe that has a mountain behind it'. Find frequency of the letter 'd'

Ex Output:
1