Calculation question 1-2 + 3-4 + 5-6... + 2005-2006

Calculation question 1-2 + 3-4 + 5-6... + 2005-2006

1-2+3-4+5-6...+2005-2006
=(1-2) + (3-4) +... + (2005-2006) a total of 1003 groups
=-1-1 -... - 1 total 1003
=-1003