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

Ex Input:
Sentence: 'a passenger bus that is driving down the street'. Find frequency of the letter 'r'

Ex Output:
3


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: 'a baseball player ready to bat the ball'. Find frequency of the letter 'a'

Ex Output:

7