def calc_density_diff(cube1_value, cube2_value):
    """Computes formated density difference.

    Parameters
    ----------
    cube1_value : str
        Single value of electron density.
    cube2_value : str
        Single value of electron density.
    
    Returns
    -------
    str
        cube1 - cube2 value.
    
    Examples
    --------
    >>> calc_density_diff('3.73097E-15', '2.43683E-15')
    '1.29414E-15'
    """
    
    diff = float(cube1_value) - float(cube2_value)

    diff = '{:0.5E}'.format(diff)
    return str(diff)