1+2+3-4+5+6-7-8+9+10-11-12+13+14-15-16+17+18-19-20+21+22-23-24+25+26-27-28
=1+2+3+(-4+5+6-7)+(-8+9+10-11)+(-12+13+14-15)+(-16+17+18-19)+(-20+21+22-23)+(-24+25+26-27)-28 (括号里的结果全是0)
=1+2+3-28
=-22
1+2+3-4+5+6-7-8+9+10-11-12+13+14-15-16+17+18-19-20+21+22-23-24+25+26-27-28
=1+2+3+(-4+5+6-7)+(-8+9+10-11)+(-12+13+14-15)+(-16+17+18-19)+(-20+21+22-23)+(-24+25+26-27)-28 (括号里的结果全是0)
=1+2+3-28
=-22