import pandas as pd
import numpy as np

def atr(df: pd.DataFrame, period: int) -> pd.Series:
    high = df["high"]
    low = df["low"]
    close = df["close"]

    prev_close = close.shift(1)
    tr = pd.concat([
        high - low,
        (high - prev_close).abs(),
        (low - prev_close).abs()
    ], axis=1).max(axis=1)

    return tr.ewm(span=period, adjust=False).mean()