in reply to stupid question about switching signs every number
$ perl -le' $_ = join "+", 1 .. 10; print; s/\+(\d+(?:\+|$))/-$1/g; pr +int; print eval' 1+2+3+4+5+6+7+8+9+10 1-2+3-4+5-6+7-8+9-10 -5 [download]