1/(1*3)+1/(2*4)+1/(3*5)+...+1/(18*20)
=1/2*(1-1/3+1/2-1/4+1/3-1/5+1/4-1/6+...+1/16-1/8+1/17-1/19+1/18-1/20)
=1/2*(1+1/2-1/19-1/20)
=531/760