loop to regroup modality of a variable

[scidam]

As it could be seen from the context (df - variable), you are using Pandas.
So, you can create Pandas dataframe from string:

from io import StringIO
import pandas as pd

# text variable defined here

data = pd.read_table(StringIO(text), sep='\s', header=None)

Further, you can get access to the first column, e.g. data.iloc[:, 0], apply mapping to it or do anything you want.

data.iloc[2:, -1].sum() returns sum you are trying to find.