I have to use precedent row value to calculate new value. 
I have a lot of rows, i don't want to use loop because it takes a lot of time, i tried with shift but i think it doesn't help me or i probably don't know how to use it.

here, little example about what i want to do.

I have a dataframe

```
df = pd.DataFrame({'A': [4,2,3,2], 'B': [2, 3,4,5]})  
In [273]: df                                                                                   
Out[273]:  
   A  B   
0  4  2  
1  2  3  
2  3  4 
3  2  5  

```

I want to create new column 'C':

* First value of C is first value of A
* Second value of C, takes first value of C and add B (second),
* Third value of C, takes second value of C and add B (third) To get something like (final result)

`In [273]: df 
Out[273]: 
 A B C
0 4 2 4.0
1 2 3 7.0
2 3 4 11.0
3 2 5 16.0`
I really dont want to use loop or apply so if there is another fast way i really want to know it. 

Thank you very much