x = city_frame["population"].cumsum()
print(x)
country
England 8615246
Germany 12177412
Spain 15342647
Italy 18216685
France 20489990
Austria 22295671
Romania 24099096
Germany 25859529
Hungary 27613529
Poland 29353648
Spain 30956034
Germany 32449934
Italy 33800614
Name: population, dtype: int64