Question 497220
Here's the formula:
{{{(a[1]+a[n])(n/2)}}} Where: {{{a[1]}}} is the first number (That's number 1),  {{{a[n]}}} is the nth number (that's 1000), and {{{n}}} is the number of numbers (that's 1000).
So...
{{{(1+1000)(1000/2) = (1001)(500)}}}=500,500