-1/2+1/6+1/12+1/30+1/42
=-1/2+(1/2-1/3)+(1/3-1/4)+(1/5-1/6)+(1/6-1/7)
=-1/2+1/2-1/3+1/3-1/4+1/4+1/5-1/6+1/6-1/7
=1/5-1/7
=2/35