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

Example Input: Sentence: 'a man standing in front of a statue near a metal pole'. Find frequency of the letter 'f'
Example Output: 2

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: 'a girl sitting at a table with a birthday cake with candles sitting on it'. Find frequency of the letter 'l'
Example Output:
3