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

Ex Input:
Sentence: 'a young boy wearing black and holding a skateboard on the sidewalk'. Find frequency of the letter 'n'

Ex Output:
5


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: 'an owl is sitting next to its nest in a tree'. Find frequency of the letter 'l'

Ex Output:

1