python pandas:算术前填充

我有以下数据框:

            AAPL shares  GOOG shares  MSFT shares
date                                             
2019-01-01          NaN         10.0          NaN
2019-01-05          NaN          NaN         15.0
2019-01-12          NaN          NaN          7.0
2019-01-13          3.0          NaN          NaN
2019-01-14          NaN         -5.0          NaN

应用正向填充后:

print(df.set_index('date').sort_index().fillna(method='ffill').fillna(value=0))

我得到:

            AAPL shares  GOOG shares  MSFT shares
date                                             
2019-01-01          0.0         10.0          0.0
2019-01-05          0.0         10.0         15.0
2019-01-12          0.0         10.0          7.0
2019-01-13          3.0         10.0          7.0
2019-01-14          3.0         -5.0          7.0

我的问题是,有没有办法用简单的加法来补充?我想要的结果是:

            AAPL shares  GOOG shares  MSFT shares
date                                             
2019-01-01          0.0         10.0          0.0
2019-01-05          0.0         10.0         15.0
2019-01-12          0.0         10.0         22.0
2019-01-13          3.0         10.0         22.0
2019-01-14          3.0          5.0         22.0