I basically have this

```
   first    
0   bar 
1   bar 
2   foo 
3   foo 

```

```
   bar foo
0   a   b

```

I want to insert value in dataframe 2 to dataframe 1, so the final result should be something like this

```
   first    
0   a   
1   a   
2   b   
3   b   

```