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

Example Input: Sentence: 'a baby elephant walks beside an adult elephant along a post and cable'. Find frequency of the letter 'l'
Example Output: 6

Example Input: Sentence: 'a piece of bread stuffed with meat and vegetables'. Find frequency of the letter 's'
Example Output: 2

Example Input: Sentence: 'breakfast of small donuts and a cup of coffee'. Find frequency of the letter 'p'
Example Output:
1