#include <iostream>

using namespace std;

int Calc(int maxFloor,int carrent){
    static int patterns = 1;
    if(carrent + 1 < maxFloor){
        Calc(maxFloor,++carrent);
        patterns++;
    }
    if(carrent + 2 < maxFloor){
        return Calc(maxFloor,++carrent);
    }
    return patterns;
}

int main(){
    unsigned int floor = 0;
    cin >> floor;
    cout << Calc(floor,0);
}