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

Ex Input:
Sentence: 'a white police vehicle parked on a paved street'. Find frequency of the letter 'a'

Ex Output:
4


Ex Input:
Sentence: 'two women standing next to each other holding tennis rackets'. Find frequency of the letter 't'

Ex Output:
7


Ex Input:
Sentence: 'a plane in the process of landing at an airport'. Find frequency of the letter 'r'

Ex Output:

3