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

Ex Input:
Sentence: 'young girl playing with refrigerator magnet while kneeling on floor'. Find frequency of the letter 'n'

Ex Output:
6

Ex Input:
Sentence: 'a couple of buses parked on the side of the street'. Find frequency of the letter 'b'

Ex Output:
1

Ex Input:
Sentence: 'the toilet is in a room with exposed pipes'. Find frequency of the letter 'i'

Ex Output:
5