#include <cstdio>

int n;

int solve(int num)
{
   if (num == 0) {
      return 1;
   } else if (num == 1) {
      return 2;
   } else if (num == 2) {
      return 4;
   } else if (num == 3) {
      return 7;
   } else {
      int res = 0;
      res += solve(num - 1) + solve(num - 2) + solve(num - 3);
      return res;
   }
}

int main( )
{
    int n;
   std::scanf("%d", &n);
   std::printf("%d", solve(n));
}