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

Ex Input:
Sentence: 'a plastic container next to a toilet on a wooden box'. Find frequency of the letter 'i'

Ex Output:
3

Ex Input:
Sentence: 'a picture of a machine built next to a street'. Find frequency of the letter 'x'

Ex Output:
1

Ex Input:
Sentence: 'a vase with flowers, a clock and paintings inside of a building'. Find frequency of the letter 'd'

Ex Output:
3