100%

#include<bits/stdc++.h>
using namespace std;
long long a[30];
int main(){
	
	a[1] = 1;
	a[2] = 2;
	a[3] = 4;
	for(int i = 4; i <= 20;i++){
		a[i] = a[i - 1] + a[i - 2]+a[i - 3];
	}
    while(1){
        int x;
        cin >> x;
        if(x==0){
            break;
        }
        cout << a[x] << endl;
    }
	return 0;
}