import pandas as pd

def add_column(df: pd.DataFrame) -> pd.DataFrame:
    df['percentage_reached_real'] = df['usd_pledged_real']/df['usd_goal_real']

    try:
        assert df['percentage_reached_real'].isna().sum() == 0
    except AssertionError as e:
        print("Adding the percentage_reached_real column has faield")
        print(e)
        
    return df
