1998*(1/1*2 +1/2*3 +1/3*4+ 1/4*5 +1/5*6) =1998*(1 -1/2 +1/2 -1/3 +1/3 -1/4 +1/4 -1/5+1/5 -1/6) =1998*(1 -1/6) =1998*5/6 =1665
