Some 100% people can't do it. What is the sum of all these natural numbers from 1999 to 999 1?

1999,2000,200 1……9989,9990,999 1

1999+999 1= 1 1990

2000+9990= 1 1990

And so on (1999+999 1, 2000+9990 ...)

(9990-1999)/2 = 7001/2 cannot be deleted.

So there is an extra number between 1999 and 999 1.

(9990+ 1999)/2=5995

Test1999 (+3996) 5995 (+3996) 9991.

They are all short of 3996, and the middle is 5995.

Then, (7001-1)/2 = 350011990.

3500* 1 1990+5995=4 1970995