Start by naming any country that France borders. Then name any country that borders any of those countries. Then keep going until there are no more countries left to name.
Actually, Shanghai is the most populated city at 26 million and Beijing is next at 20 million. I am not including the outskirts of the cities. For example, if I did, Guangzhou would be first because Shenzen is in its outskirts. However, I am not doing that.