Difference function

Given a City table, whose fields are described as

+-------------+----------+
| Field | Type |
+-------------+----------+
| ID | int(11) |
| Name | char(35) |
| CountryCode | char(3) |
| District | char(20) |
| Population | int(11) |
+-------------+----------+

print the difference between the maximum and minimum city populations.

>>select (max(population)-min(population)) from city;

Thanks.