Python3（https://docs.python.org/3/library/math.html#math.floor）：
math.floor(x)
Return the floor of x, the largest integer less than or equal to x. If x is not a float, delegates to x.__floor__(), which should return an Integral value.
Python2（https://docs.python.org/2/library/math.html#math.floor）：
math.floor(x)
Return the floor of x as a float, the largest integer value less than or equal to x.