#include<bits/stdc++.h>
using namespace std;
int main(){
    int n;
    cin>>n;
    int a,b,c,x;
    a=1;
    b=2;
    c=4;
    if(n==1){
        cout<<1<<endl;
    }
    else if(n==2){
        cout<<2<<endl;
    }
    else if(n==3){
        cout<<4<<endl;
    }
    else{
     for(int i=4;i<=n;i++){
        x=a+b+c;
        a=b;
        b=c;
        c=x;
     }   
     cout<<x;
    }
}
/**************************************************************
	Problem: 1367
	User: luyanchen
	Language: C++
	Result: Accepted
	Time:22 ms
	Memory:2072 kb
****************************************************************/