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

Example Input: Sentence: 'an area that has surrounded by a metal fence'. Find frequency of the letter 'o'
Example Output: 1

Example Input: Sentence: 'this marketplace has a lot of fresh fruit and vegetables'. Find frequency of the letter 'v'
Example Output: 1

Example Input: Sentence: 'a man crossing the street on a busy road'. Find frequency of the letter 'a'
Example Output:
4