def formula(df):
    formula = np.zeros([len(df.period.unique()),1])
    for i in range(len(period.unique())):
        count = 0
        amount = 0
        dt = df[df.datetime==fd.period.unique()[i]]
        for j in range(len(dt)):
            if count + dt.amount[j] <= 500:
                count = count + dt.amount[j]
                priceamount = amount + dt.amount[j]*dt.price[j]
            else:
                diff = 500-count
                priceamount = priceamount + diff*dt.price[j]
                formula[i] = priceamount/500
                break
    return formula