C++ Variable Change a Variable after assigning a value

#include <iostream> 

using namespace std; 

int main() //from  w  ww  . jav  a 2 s .c o  m
{ 
  int mynumber; 
  mynumber = 10; 
  cout << mynumber << endl; 
  mynumber = 20; 
  cout << mynumber << endl; 
  return 0; 
}