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

Ex Input:
Sentence: 'a truck is parked in front of a store'. Find frequency of the letter 't'

Ex Output:
3

Ex Input:
Sentence: 'a social occasion of people are uncovered before the open day'. Find frequency of the letter 'c'

Ex Output:
4

Ex Input:
Sentence: 'a clown, teddy bear and troll doll for sale in a store'. Find frequency of the letter 'n'

Ex Output:
3