走楼梯2（Daimayuan Online Judge）

/*********************************************************************
    程序名:
    版权: Joecai
    作者: Joecai
    日期: 2022-03-27 21:17
    说明:
*********************************************************************/
#include <bits/stdc++.h>
using namespace std;
#define x first
#define y second
# define rep(i,be,en) for(int i=be;i<=en;i++)
# define pre(i,be,en) for(int i=be;i>=en;i--)
typedef pair<int, int> PII;
#define ll long long
#define endl "\n"
#define LOCAL
#define pb push_back
const int N = 2e5 + 10, INF = 0x3f3f3f3f;
int n;
ll f[100][3];
void solve()
{
  cin >> n;
  f[0][0] = 1;
  f[1][0] = 1;
  for (int i = 2; i <= n; i++)
  {
    f[i][0] += f[i - 1][0] + f[i - 1][2] + f[i - 1][1];
    f[i][1] += f[i - 2][0];
    f[i][2] += + f[i - 2][1];
  }
  cout << f[n][0] + f[n][1] + f[n][2] << endl;
}
int main() {
  //std::ios::sync_with_stdio(false);
  //std::cin.tie(nullptr);
  //#ifdef LOCAL
  //freopen("data.in.txt","r",stdin);
  //freopen("data.out.txt","w",stdout);
  //#endif
  int __ = 1;
  //cin>>__;
  while (__--)
  {
    solve();
  }
  return 0;
}