1/1x2+1/2x3+1/3x4+...+1/(n-1)xn=2013/2014
1/1-1/2+1/2-1/3+1/3-1/4+...+1/(n-1)-1/n=2013/2014
1/1-1/n=2013/2014
(n-1)/n=2013/2014
n=2014