-1/2+1/6+1/12+1/20+1/30+1/42
=-1/2+1/2*3+1/3*4+1/4*5+1/5*6+1/6*7
=-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=-1/7