def validate_orders(df: pd.DataFrame) -> None:
assert len(df) > 0, "empty dataframe"
# 키 유일성
dup = df["order_id"].duplicated().mean()
assert dup == 0, f"duplicated order_id: {dup:.4f}"
# 결측률
miss = df[["order_id", "amount", "region"]].isna().mean()
assert miss["amount"] < 0.01, f"amount missing too high: {miss['amount']:.4f}"
# 범위
assert (df["amount"] >= 0).all(), "negative amount detected"
# 허용값
allowed = {"seoul", "busan", "incheon"}
invalid = set(df["region"].dropna().unique()) - allowed
assert not invalid, f"invalid region values: {invalid}"