9999＋9997＋…＋9001-1＋3＋…＋999

First, let's find the sum of the first 500 odd numbers (1, 3, 5, ... , 999): n = 500 sum = n² = 500² = 250,000

Next, let's find the sum of the first 500 even numbers (2, 4, 6, ... , 1000): n = 500 sum = n(n+1) = 500(500+1) = 250,500

Now we can subtract the sum of the odd numbers from the sum of the even numbers: 250,500 - 250,000 = 500

Finally, let's find the sum of the integers from 9001 to 9999 with a common difference of 2: a₁ = 9001 aₙ = 9999 d = 2 n = (aₙ - a₁)/d + 1 = (9999 - 9001)/2 + 1 = 500 sum = n(a₁ + aₙ)/2 = 500(9001 + 9999)/2 = 9,000,000

Now we can subtract the sum of the odd numbers from the sum of the even numbers and the sum of the integers from 9001 to 9999: 9,000,000 - 500 = 8,999,500

Therefore, (9999 + 9997 + ... + 9001) - (1 + 3 + ... + 999) = 8,999,500.