#include<stdio.h>
FILE*f=fopen("nunta.in","r");
FILE*g=fopen("nunta.out","w");
long long unsigned n;
int main()
	{
	long long unsigned c,i,a,b;
	fscanf(f,"%llu",&n);
	a=1;
	b=2;
	for(i=3;i<=n;++i)
		{
		c=a+b;
		a=b;
		b=c;}

	fprintf(g,"%llu",c);
	return 0;
	}