MR-864 / 4b. problem

Simplify the following expression:

1-1+2-2+3-3+-1-1+-2-2+-3-3

1-1+2-2+3-3+-1-1+-2-2+-3-3

=111+122+133+-111+-122+-133

=11+14+127-11+14-127

=14+14

=24

=12