def normalize(img, mean, std):
    """
    Normalize image with mean and standard deviation.
    
    Parameters
    ----------
    img : array(float)
        The image to normalize
    mean : float
        The mean used for normalization.
    std : float
        The standard deviation used for normalization.
        
    Returns
    -------
    array(float)
        The normalized image.
    """
    return (img - mean) / std