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

Example Input: Sentence: 'a man is water skiing on a river'. Find frequency of the letter 'r'
Example Output: 3

Example Input: Sentence: 'an audience watches as a bird spreads its wings'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'a plane flying low above a large amount of water'. Find frequency of the letter 'l'
Example Output:
4