时间复杂度大概是
∑d=1n∑k=1⌊nd⌋⌊nkd⌋\sum_{d=1}^n\sum_{k=1}^{\lfloor\frac{n}{d}\rfloor}\lfloor\dfrac{n}{kd}\rfloor∑d=1n∑k=1⌊dn⌋⌊kdn⌋
