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

Example Input: Sentence: 'a man with headphones on is hunched over a sink while being recorded'. Find frequency of the letter 'd'
Example Output: 4

Example Input: Sentence: 'a couple of girls are hanging out at a restaurant'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'a view of very old ships in a harbor, with building behind them'. Find frequency of the letter 'd'
Example Output:
3