#include <iostream>
using namespace std;
typedef long long LL;

int main() {
    LL f[21] = {0, 0, 1}; //这个初始化很牛B的样子,放过头雁打二雁，而且f[1]=0,f[2]=1
    //错排公式,预处理
    for (int i = 3; i < 21; i++) f[i] = (i - 1) * (f[i - 1] + f[i - 2]);

    for (int i = 1; i <= 20; i++)
        cout << "i=" << i << ",f[" << i << "]=" << f[i] << endl;
    return 0;
}