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

Ex Input:
Sentence: 'cars parked outside if a retail store'. Find frequency of the letter 'd'

Ex Output:
2

Ex Input:
Sentence: 'three young children playing with a ball in front of a huge bridge'. Find frequency of the letter 'e'

Ex Output:
5

Ex Input:
Sentence: 'a laptop on a stool outside the field'. Find frequency of the letter 'f'

Ex Output:
1