#include<iostream.h>
/*
TASK:necklace
LANG:C++
*/
long n,a[101],i;
int main()
{
 a[2]=1;
 a[3]=3;
 cin>>n;
 if(n<4) cout<<a[n]<<endl;
 else
 {
  for(i=4;i<=n;i++)
  {
   a[i]=a[i-1]+a[i-2];
  }
   cout<<a[i-1]<<endl;
 }
return 0;
}