```c++
#include <iostream>
int main() {
std::cout << "Hello World!\n";
int i,n,s;
std::cout<<"输入n";
std::cin>>n;
i=1;
s=1;
while(i<n)
{
s*=i;
i++;
}
std::cout<<n<<"!="<<s<<std::endl;
}
```