import math


def simpFDI(temp, humid, wind, df):
    """
        Description
        -----------
                Calculates the Macarthur FDI via a simple formula.

        Parameters
        ----------
                temp  : float
                humid : float
                wind  : float
                df    : integer

        Returns
        -------
                FDI : float
    """
    return 2*(math.exp((0.987*math.log(df+0.001))-.45-(.0345*humid)+(.0338*temp)+(.0234*wind)))