x1=1 x2=2 x3=3.x2003=2003
2^x1-2^x2-2^x3-…-2^x2003
=2^1-2^2-2^3-.-2^2003
=2-2*(2+2^2+.2^2002)
=2-2*2*(2^2002-1)/(2-1)
=2-4*2^2002+4
=6-2^2004