Problem Solution

The diagram shows that $1 + 3 + 5 + 7 + 5 + 3 + 1 = 3^2 + 4^2$.

What is $1 + 3 + 5 + \dots + 1999 + 2001 + 1999 + \dots + 5 + 3 + 1$?

If you liked this problem, here is an NRICH task which challenges you to use similar mathematical ideas.