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

Example Input: Sentence: 'a gourmet looking hamburger with fries on a dining table with some wine'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a woman fully dressed as a geisha'. Find frequency of the letter 'm'
Example Output: 1

Example Input: Sentence: 'an airplane sitting on the tarmac under a night sky'. Find frequency of the letter 'n'
Example Output:
6