I have a dataframe like the following. 

```
A = [{'ID':1, 'Period':1, 'Variable':21}, {'ID':1,'Period':2, 'Variable':12}, 
      {'ID':2, 'Period':2, 'Variable':14}, {'ID':2, 'Period':3, 'Variable':18}]

df = pd.DataFrame(A)

```

I would essentially like to do an operation like `df.groupby('ID').sum()` to get the sum of the `Variable` column, but I need to skip the first period observed for a particular ID. So, for ID=1, I am dropping the observation at period 1, but for ID=2, I am dropping the observation at period 2. 

How can I do this?