def METHOD_NAME(x_arr, y_arr):
    """Calculate slope."""
    xy_sum = (x_arr * y_arr).sum()
    xx_sum = (x_arr * x_arr).sum()
    x_mean = x_arr.mean()
    slope = (xy_sum - x_mean * y_arr.sum()) / (xx_sum - x_mean * x_arr.sum())
    return slope