#include <iostream> using namespace std; long long answer(long long n) { if (n == 1) { return 0; } else if (n == 2) { return 1; } else if (n == 3) { return 1; } else { return answer(n - 3) + 2 * answer(n - 2) + answer(n - 1); } } int main() { long long number; cin >> number; long long ans = answer(number); cout << ans << endl; return 0; } //这道题还行,1用递归还给了公式