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

Example Input: Sentence: 'a train is sitting at a train station'. Find frequency of the letter 'r'
Example Output: 2

Example Input: Sentence: 'a bird on a tree is flapping its wings'. Find frequency of the letter 'o'
Example Output: 1

Example Input: Sentence: 'a bear runs through the verdant woods'. Find frequency of the letter 'r'
Example Output:
4