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

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 dog sitting in the back of a truck flat bed'. Find frequency of the letter 'h'
Example Output: 1

Example Input: Sentence: 'two bears are swimming in a body of water'. Find frequency of the letter 'a'
Example Output:
4