...

#include<bits/stdc++.h>
using namespace std;
int a[100005];
int main(){
int n;
cin >> n;
a[1] = 1;
a[2] = 2;
a[3] = 4;
for(int i = 4;i <= n;i++) a[i] = a[i-1] + a[i-2] + a[i-3];
cout << a[n];
return 0;
}