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

Example Input: Sentence: 'a bathroom with a sink, counter top, and mirror is shown'. Find frequency of the letter 'i'
Example Output: 4

Example Input: Sentence: 'a man in costume talking on a cell phone'. Find frequency of the letter 'm'
Example Output: 2

Example Input: Sentence: 'a train is comign down the trakc with people in the last wagon'. Find frequency of the letter 'o'
Example Output:
4