请看一下，基础题

把static int sum; 改为非静态程序应该怎么改呢？
//sample.h
#include <iostream.h>
class Sample
{
public:
      Sample(int x):n(x){}
      void add(){sum+=n;}
      void disp(){cout<<"n="<<n<<", sum="<<sum<<endl;}
private:
      int n;
      static int sum;
};
//test.cpp
#include <iostream.h>
#include "sample.h"
int Sample::sum=0;
void main()
{
      Sample a(2), b(3), c(5);
      a.add();
      a.disp();
      b.add();
      b.disp();
      c.add();
      c.disp();
}