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

Example Input: Sentence: 'a dog on a couch near many pillows'. Find frequency of the letter 'p'
Example Output: 1

Example Input: Sentence: 'a herd of giraffes in tall african grass'. Find frequency of the letter 'a'
Example Output: 6

Example Input: Sentence: 'a plate with a hot dog, chips and a strawberry'. Find frequency of the letter 'a'
Example Output:
6