We can calculate the sum of all the columns of a DataFrame or the sum of certain columns:
city_frame.sum())
city_frame["population"].sum()
33800614
name LondonBerlinMadridRomeParisViennaBucharestHamb...
country EnglandGermanySpainItalyFranceAustriaRomaniaGe...
population 33800614
dtype: object