(1/2-1)*(1/3-1)*(1/4-1)*……*(1/20-1)
=(-1/2)*(-2/3)*...*(-19/20)
=-(1/2)*(2/3)*...*(19/20)
=-1/20
-4*10^2=a*10^n
a=-4
n=2
a+n=-4+2=-2