#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(){ long long n,a[41]={0,1,2,4}; cin>>n; for(long long i=4;i<=n;i++){ a[i]=a[i-1]+a[i-2]+a[i-3]; } cout<<a[n]; return 0; }