#include<bits/stdc++.h>
using namespace std;
int ways(int x);
int main()
{
    int x;
    cin>>x;
    cout<<ways(x)<<endl;
    return 0;
}

int ways(int x)
{
    if(x==1)
        return 1;
    else
    {
        if(x==2)
            return 2;
        else
        {
            if(x==3)
                return 4;
            else
                return ways(x-1)+ways(x-2)+ways(x-3);
        }
    }
}