原式=（1-1/2）（1+1/2）（1-1/3）（1+1/3）（1-1/4）（1+1/4）...
=1/2x3/2x2/3x4/3x3/4x5/4...
=1/2x2/3x3/4...x3/2x4/3x5/4...
=1/2014x2015/2
=2015/4028