//4_10

#include <iostream.h>

void main()
{
  int n;
  long a=1, b=1, c=1, temp;
  cout <<"please input a value:\n";
  cin >>n;

  for(int i=4; i<=n; i++){
    temp=a+c; a=b; b=c; c=temp;
  }
  cout << c <<endl;
}