#include <fstream> 
using namespace std;

ifstream f("nunta.in"); ofstream g("nunta.out");

int a, b, c, n, i;

int main(){
	f>>n; a=1; b=2;
	
	for (i=3; i<=n; i++){
		c=a+b;
		a=b; b=c;
	}
	
	if (n==1) g<<1;	else g<<b;
}