如何让Pandas从下一行开始在新列中创建sumsum？

import pandas as pd

df = pd.DataFrame({'A': [1, 2, 3, 4, 5]})

df['sumsum'] = df['A'].shift(1)

df['sumsum'] = df['sumsum'].fillna(0) + df['A']

import pandas as pd

df = pd.DataFrame({'A': [1, 2, 3, 4, 5]})
df['sumsum'] = df['A'].shift(1)
df['sumsum'] = df['sumsum'].fillna(0) + df['A']

print(df)

   A  sumsum
0  1     0.0
1  2     3.0
2  3     5.0
3  4     7.0
4  5     9.0