程序代码：

unsigned foo( unsigned n )
{
    unsigned r = 0;
    for( unsigned i=0,t=1; i!=n; t*=++i+1 )
        r += t;
    return r;
}

#include <iostream>
using namespace std;

int main( void )
{
    cout << foo(0) << endl;
    cout << foo(1) << endl;
    cout << foo(5) << endl;
}