(1/2-1)x(1/3-1)x(1/4-1)...(1/48-1)x(1/49-1)x(1/50-1)
=-(1-1/2)(1-1/3)(1-1/4)*.*(1-1/48)(1-1/49)(1-1/50
=-1/2*2/3*3/4*4/5*...*47/48*48/49*49/50
=-1/50