从左往右运算
2-1+52-21+2-10+2-1-15
=1+52-21+2-10+2-1-15
=53-21+2-10+2-1-15
=32+2-10+2-1-15
=34-10+2-1-15
=24+2-1-15
=26-1-15
=25-15
=10