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

Example Input: Sentence: 'a blue tripocal bird is perched on a wooden fence'. Find frequency of the letter 'd'
Example Output: 3

Example Input: Sentence: 'two elephants walking on cement next to wall'. Find frequency of the letter 'n'
Example Output: 5

Example Input: Sentence: 'a desktop computer sitting on a wooden desk'. Find frequency of the letter 'e'
Example Output:
4