500,500.

It's easiest to see that the first and last number are 1 and 1000, which add to 1001.

The next two numbers we can add are 2 and 999, which add to 1001.

The next two numbers to add are 3 and 998, which add to 1001.

We can keep doing this 500 times, and in each case they add to 1001.

And 500 x 1001 = 500,500.