A tetrahedron is a pyramid with a triangular base and three sides. A tetrahedral number is a number of items within a tetrahedron.
Create a function that takes an integer n and returns the nth tetrahedral number.

Tetra(2) ➞ 4
Tetra(5) ➞ 35
Tetra(6) ➞ 56There is a formula for the nth tetrahedral number.