#include <stdio.h>

FILE *f,*g;
int n,i;
long long a,b,c;

int main()
{
  f=fopen("nunta.in","r");
  g=fopen("nunta.out","w");
  fscanf(f,"%d\n",&n);
  a=1;
  b=2;
  if((n==1)||(n==2))
    fprintf(g,"2\n");
  else
    {
      for(i=3;i<=n;i++)
	{
	  c=a+b;
	  a=b;
	  b=c;
	}
      fprintf(g,"%ld\n",b);
    }

  fclose(f);  fclose(g);
  return 0;
}