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

Example Input: Sentence: 'an old woman is holding her cat in front of a camera'. Find frequency of the letter 'r'
Example Output: 3

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: 'several boats docked on the marina with passengers wating'. Find frequency of the letter 'a'
Example Output:
6