double u=1e9,d=1e-9,m;long k,s,a[1<<18],i;main(t){for(;~scanf("%ld",&s);k=s)a[~i--]=k;for(;t++<99;s>k?d=m:(u=m))for(s=i=1;a[i];s+=a[i++]/m)m=(u+d)/2;printf("%.9f",u);}