Triangular numbers are so called because of the equilateral triangular shape that they occupy when laid out as dots. i.e.
1st (1) 2nd (3) 3rd (6)
* ** ***
* **
*
You need to return the nth triangular number. You should return 0 for out of range values:
triangular(0)==0,
triangular(2)==3,
triangular(3)==6,
triangular(-10)==0