添加两个索引不相等的数据帧

Question

a：

   Length  10kN
0      0.0     5
1      0.5     5
2      1.0     5
3      1.5     5
4      2.0     5
5      2.5     5
6      3.0     5
7      3.5     5
8      4.0     5
9      4.5     5
10     5.0     5
11     5.0    -5
12     5.5    -5
13     6.0    -5
14     6.5    -5
15     7.0    -5
16     7.5    -5
17     8.0    -5
18     8.5    -5
19     9.0    -5
20     9.5    -5
21    10.0    -5

b：

    Length1  20kN
0       0.0    50
1       0.5    45
2       1.0    40
3       1.5    35
4       2.0    30
5       2.5    25
6       3.0    20
7       3.5    15
8       4.0    10
9       4.5     5
10      5.0     0
11      5.5    -5
12      6.0   -10
13      6.5   -15
14      7.0   -20
15      7.5   -25
16      8.0   -30
17      8.5   -35
18      9.0   -40
19      9.5   -45
20     10.0   -50

c由于我的代码如下：

    Length  10kN  Length1  20kN  Total
0      0.0     5      0.0  50.0   55.0
1      0.5     5      0.5  45.0   50.0
2      1.0     5      1.0  40.0   45.0
3      1.5     5      1.5  35.0   40.0
4      2.0     5      2.0  30.0   35.0
5      2.5     5      2.5  25.0   30.0
6      3.0     5      3.0  20.0   25.0
7      3.5     5      3.5  15.0   20.0
8      4.0     5      4.0  10.0   15.0
9      4.5     5      4.5   5.0   10.0
10     5.0     5      5.0   0.0    5.0
11     5.0    -5      5.5  -5.0  -10.0
12     5.5    -5      6.0 -10.0  -15.0
13     6.0    -5      6.5 -15.0  -20.0
14     6.5    -5      7.0 -20.0  -25.0
15     7.0    -5      7.5 -25.0  -30.0
16     7.5    -5      8.0 -30.0  -35.0
17     8.0    -5      8.5 -35.0  -40.0
18     8.5    -5      9.0 -40.0  -45.0
19     9.0    -5      9.5 -45.0  -50.0
20     9.5    -5     10.0 -50.0  -55.0
21    10.0    -5      NaN   NaN    NaN

我试过的代码：

import pandas as pd
a=pd.read_csv("first.csv")
b=pd.read_csv("second.csv")
c=pd.concat([a,b], axis=1)
c['Total']=c['10kN']+c['20kN']
print(c['Total'])
print(a)
print(b)
print(c)

我想增加两列10kN和20kN，长度相同