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

Ex Input:
Sentence: 'a green bus driving in a street filled with heavy traffic'. Find frequency of the letter 'v'

Ex Output:
2


Ex Input:
Sentence: 'an commercial airline airplane flies above the clouds'. Find frequency of the letter 'n'

Ex Output:
3


Ex Input:
Sentence: 'a bird digging the wet dirt at the edge of the beach'. Find frequency of the letter 't'

Ex Output:

6