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

Example Input: Sentence: 'a room with a big screen television and a couch'. Find frequency of the letter 'h'
Example Output: 2

Example Input: Sentence: 'a parking meter painted green with a sign that describes it as not a parking meter'. Find frequency of the letter 's'
Example Output: 4

Example Input: Sentence: 'a bus is driving on the highway near other vehicles'. Find frequency of the letter 'd'
Example Output:
1