2005 Challenge Problem #3
On the first day of Christmas, my true love gave to me: a partridge in a pair tree. That's one item.
On the second day of Christmas, my true love gave to me: two turtle doves and a partridge in a pair tree.
That's three more items, for a total of four. Continuing in this fashion, my true love gives me six items on
the third day, for a total of ten. By the end of the Nth day, what is the total number of items my true love
has given me? Write the method NthDay (signature below) that returns this value. If the input value
is not positive, return -1.
public int NthDay(int N)
// fill in here