Country	Name	_Population_	_Elevation_
AD	les Escaldes	15853	1033
AD	Andorra la Vella	20430	1037
...	...	...	...
ET	Addis Ababa	2757729	2405
ET	Ādīgrat	65000	2451
...	...	...	...
MX	Mexico City	12294193	2240
...	...	...	...

db.cities.createIndex( { population: 1, elevation: 1 } );