c++中N阶乘的求法

1！=+2！=3！+4！........+20!怎么在C++中编写求以上值的程序
二楼的兄弟，好像不行，结果是210，怎么可能！ 是求1！+2！+3！+4！+5！......+20！＝？
VISUAL C++ 来又怎么写呢?

#include <iostream.h>
void main(){
int sum=0;
int num=1;
for(int i=1;i<=20;i++)
{
for(int j=i;j>=1;j--)
{
num=num*j;
}
sum+=num;
num=1;
}
cout<<"The result is "<<sum<<endl;
}

#include <iostream>
using namespace std;
int main()
{
int sum = 0;
int n = 1;
int f = n;
for(int i = 1; i <= 20; ++i,++n)
{
for(int k = 1; k < n; ++k)
{
f = n;
f = f*(f-k)
}
sum += f;
}
cout << sum << endl;
system("pause");
return 0;
}

#include <iostream>

long factorialSum( int n );

int main()
{
using std::cout;
using std::endl;

cout<<factorialSum(3)<<endl;

return 0;
}

long factorialSum( int n )