# import necessary libraries
import pandas as pd

# create your sample series
series = pd.Series([1, 3, 6, 10, 15, 21])

# find diff of diff by list comprehension
differences = [series[i+1] - series[i] 
               for i in range(len(series)-1)]
diff_of_diffs = [differences[i+1] - differences[i] 
                 for i in range(len(differences)-1)]

# print results
print(diff_of_diffs)