\(1+\left(-2\right)+3+\left(-4\right)+...+\left(-2002\right)+2003\)
\(=\left[1+\left(-2\right)\right]+\left[3+\left(-4\right)\right]+...+\left[2001+\left(-2002\right)\right]+2003\)
\(=\left(-1\right)+\left(-1\right)+...+\left(-1\right)+2003\) (1001 số -1)
= \(-1001+2003=1002\)