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

Example Input: Sentence: 'a snow filled mountain with a guy with ski's'. Find frequency of the letter 'f'
Example Output: 1

Example Input: Sentence: 'people fishing and enlarge mountainlake with trees lining the shoreline'. Find frequency of the letter 't'
Example Output: 4

Example Input: Sentence: 'a donut with a bite missing sits on the table in front of a glass of water'. Find frequency of the letter 'i'
Example Output:
6