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

Example Input: Sentence: 'a green bus driving in a street filled with heavy traffic'. Find frequency of the letter 'v'
Example Output: 2

Example Input: Sentence: 'seagulls sit on the posts of a seaside balcony by a clock tower'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'an old world war ii plan has been refurbished and modernized'. Find frequency of the letter 'n'
Example Output:
5