#include<iostream> // cin\cout\endl #include<cstdio> //scanf()\printf() #include<cstring> // strcpy()\strcat()\strcmp()\strlen()\memset() #include<cmath> //sqrt()\pow()\abs()\ceil()\floor()\max()\min() using namespace std; int main(){ int n; long long a[41] = {0,1,2,4}; for(int i = 4;i <= 40;i++) a[i] = a[i-1]+a[i-2]+a[i-3]; cin>>n; cout<<a[n]; return 0; }