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

[EX Q]: Sentence: 'a silver train crossing the road on tracks'. Find frequency of the letter 'r'
[EX A]: 5

[EX Q]: Sentence: 'two people under an umbrella while walking down a sidewalk'. Find frequency of the letter 'i'
[EX A]: 3

[EX Q]: Sentence: 'an empty park with mature trees and a backless bench'. Find frequency of the letter 'a'
[EX A]:
6