Geolocation in a dataframe

I have a data frame. with the columns cities, latitude and longitude the cities some have NaN others are complete. How can I do to fill in the columns of cities with longitude and latitude. example

Mundo = {
        'ciudades': ['Nan', 'Buenos Aires', np.NaN,'Brasil','Canada'],
        'Lon': ['34.1577057', 'NaN', '22.1577057','','62.1577057'],
        'Lat': ['-95.2731303', 'NaN', '-102.2731303','','-80.2731303']
       }

df = pd.DataFrame(Mundo)

The idea is that you can fill in the empty rows of the cities column using your coordinates thanks