Write a method LeastCommon that accepts a Dictionary whose keys are strings and whose values are integers as a parameter and returns the integer value that occurs the fewest times in the dictionary.
For example, if a dictionary named m contains {{"Alyssa", 22}, {"Char", 25}, {"Dan", 25}, {"Jeff", 20}, {"Kasey", 20}, {"Kim", 20}, {"Mogran", 25}, {"Ryan", 25}, {"Stef", 22}}, the call of LeastCommon(m) returns 22.
If there is a tie, return the smaller integer value.
If the dictionary is empty, throw an ArgumentException.
