Seeking 
(n <= 1e14)
If the direct enumeration d added together it is clearly not enough, consider optimizing.
Because it is rounded down, then ⌊m / d ⌋ may be equal ⌊m / (d + 1) ⌋, since it is the same, why should a plus instead of a plus it?
One conclusion: ⌊n / D ⌋ only O (√n) th value
