I have two datframes that look like this

```
df1

posting_period      name        sales       profit
    1               client1     50.00       10.00
    1               client2     100.00      20.00
    2               client1     150.00      30.00

df2 (this df does not have the 'profit' column as in df1) 

posting_period      name        sales       
    1               client1     10.00       
    2               client1     20.00   

```

I want to update the sales in `df1` for client1 with the sum of client1's sales in `df1` and client1's sales in `df2` where the posting\_periods match. In other words

```
desired result

posting_period      name        sales       profit
    1               client1     60.00       10.00
    1               client2     100.00      20.00
    2               client1     170.00      30.00

```

The actual dataframes I'm working with are much larger, but these examples capture what I'm trying to accomplish. I came up with a very round-about way that not only didn't work, it wasn't very pythonic. The other challenge is the additional column in `df1` not in `df2`. I was hoping someone could suggest an alternative. Thank you!