(1m+3m+5m+...+2009m)-(2m+4m+6m+...+2008m)
=1m+3m+5m+...+2009m-2m-4m-6m-...-2008m
=1m+(3m-2m)+(5m-4m)+...+(2009m-2008m)
=1m×1005
=1005m