double u=1e10,d=1e-10,m;long k,s,a[200010],i;main(t){for(;~scanf("%ld",a+s);k=a[s++]);for(;t++<99;s>k?d=m:(u=m))for(s=i=1;a[i]!=k;s+=a[i++]/m)m=(u+d)/2;i=!printf("%.9f",u);}