填空题 若有以下程序：
#include ＜iostream＞
using namespace std;
class Base

public:
Base()

x=0;

int x;
;
class Derived1: virtual public Base

public:
Derived1()

x=10;

;
class Derived2: virtual public Base

public:
Derived2()

x=20;

;
class Derived: public Derived1, protected Derived2
;
int main()

Derived obj;
cout＜＜obj.x＜＜end1;
return 0;

该程序运行后的输出结果是 【15】 。