20+19-18-17+16+15-14+13+12+11-10-9
=(20+19-18-17)+(16+15-14+13)+(12+11-10-9)
=(20-18+19-17)+(16-14+15+13)+(12-10+11-9)
=(2+2)+(2+2)+(2+2)
=4+4+4
=12