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

Example Input: Sentence: 'a piece of film shows two street signs crossing each other'. Find frequency of the letter 'f'
Example Output: 2

Example Input: Sentence: 'the man is holding a plastic sled with snow in it next to a child'. Find frequency of the letter 'g'
Example Output: 1

Example Input: Sentence: 'a toilet of some sort near a sink of some sort'. Find frequency of the letter 'o'
Example Output:
7