If you would like to calculate 7 + 9 + 11 + ... + 2011 + 2013 - 5 - 7 - 9 - 11 - ... - 2011, you can do this using the following steps:
7 + 9 + 11 + ... + 2011 + 2013 - 5 - 7 - 9 - 11 - ... - 2011 = (7 + 9 + 11 + ... + 2011) + 2013 - 5 + (- 7 - 9 - 11 - ... - 2011) = 2013 - 5 = 2008
The correct result is 2008.