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

Example Input: Sentence: 'a bird in a tree looking up at branches'. Find frequency of the letter 'e'
Example Output: 3

Example Input: Sentence: 'a bedroom with a neatly made bed and a laptop'. Find frequency of the letter 'l'
Example Output: 2

Example Input: Sentence: 'a newborn baby covered with a blanket sleeping'. Find frequency of the letter 'b'
Example Output:
4