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

Ex Input:
Sentence: 'a group of people walking down a rain soaked street with umbrellas'. Find frequency of the letter 'w'

Ex Output:
3

Ex Input:
Sentence: 'a man standing on a tennis court holding a racquet'. Find frequency of the letter 's'

Ex Output:
2

Ex Input:
Sentence: 'the girl is smiling beside her tired horse'. Find frequency of the letter 'i'

Ex Output:
6