Wal-Mart is how big?

According to the NY Times, the annual sales of Wal-Mart have topped US$400.000.000.000, or $400 billion. By my calculations, that means that if it were a country, Wal-Mart's gross domestic product (GDP), measured solely by its annual sales (I know it's a bad measure, as GDP = C + I + G + (X - M), where

C = private spending
I = business investment (facilities, equipment, etc)
G = government spending
X = exports
M = imports )

would make it the 18th largest country, as measured by GDP, with more economic output than Austria, Norway, Chile, or Indonesia. (We know that much of what Wal-Mart sells is imported (meaning M is large), so the figures don't work exactly. But humor me here!)