| A Short Introduction to Operating System Recent countries visiting this post |  |
 | 46 | United States |
 | 31 | |
 | 18 | China |
 | 6 | Netherlands |
 | 5 | Vietnam |
 | 5 | Germany |
 | 3 | Czech Republic |
 | 2 | France |
 | 1 | Russian Federation |
 | 1 | Europe |
 | 1 | Italy |
 | 1 | Turkey |
 | 1 | Colombia |
 | 1 | Philippines |
 | 1 | United Kingdom |