#include<fstream>
using namespace std;
ifstream f("nunta.in");
ofstream g("nunta.out");
int n,i;
unsigned long long a=1,b=2,c;
int main()
{ f>>n;
  for(i=3;i<=n;++i) { c=a+b;
                      a=b;
                      b=c;
                     }
  g<<c<<'\n';
  return 0;
}