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

Ex Input:
Sentence: 'a herd of zebra standing on top  of a grass covered field'. Find frequency of the letter 'e'

Ex Output:
5


Ex Input:
Sentence: 'a big bear walks through some grass'. Find frequency of the letter 'a'

Ex Output:
4


Ex Input:
Sentence: 'a plane is on the ground, it looks dirty'. Find frequency of the letter 'y'

Ex Output:

1