全错位排列

//C++示例代码
#include <iostream>
using namespace std;

long long getvalue(int num)
{
    if (num == 1)
    {
        return 0;
    }
    else if (num == 2)
    {
        return 1;
    }
    else if (num == 3)
    {
        return 2;
    }
    else
    {
        long long f1 = 1;
        long long f2 = 2;
        for (int i = 4; i <= num; i++)
        {
            long long t = f2;
            f2 = (i - 1)*(f1 + f2);
            f1 = t;
        }
        return f2;
    }
}

int main()
{
    int num;
    cout << "input the number of envelop with -1 to end" << endl;
    while (cin>>num)
    {
        if (num == -1)
            break;
        long long r = getvalue(num);
        cout<<"Result:" << r << endl;
    }
    return 0;
}