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

Example Input: Sentence: 'a child sitting a table with a pizza in front of him'. Find frequency of the letter 'h'
Example Output: 3

Example Input: Sentence: 'a group of people seated in chairs together and facing the same direction, with one man holding a soccer ball in his hands'. Find frequency of the letter 'm'
Example Output: 2

Example Input: Sentence: 'a dog chasing a baby cow across a lush green field'. Find frequency of the letter 'a'
Example Output:
6