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

Example Input: Sentence: 'a group of people standing on grass with a train in the background'. Find frequency of the letter 'n'
Example Output: 6

Example Input: Sentence: 'a teenager witting on a futon couch holding gaming controls in both hands'. Find frequency of the letter 'c'
Example Output: 3

Example Input: Sentence: 'two skiers standing in the snow with a landscape view behind them'. Find frequency of the letter 'n'
Example Output:
6