#include <iostream>

using namespace std;

int main()
{
    int n;
    cin>>n;
    int x[30];
    x[1] = 1;
    x[2] = 2;
    x[3] = 5;
    for(int i = 4; i <= n; i++){
        x[i] = x[i-1] + x[i-3];
    }
    cout<<x[n];
    return 0;
}