def _std(var:list, mean:float, length:int):
    """
Compute the Standar Deviation of a point

Parameters:
------------

var: list
    The variance between all points

mean: float
    The mean of the point

lenght: int
    The lenght of all routes

Return
-------
Return the standar deviation of a point
    """

    summatory = 0
    for i in range(length):
        summatory += (var[i] - mean)**2

    return (summatory / length)**0.5