class Five_Year
{
public:
  Five_Year(int y) : year((y/5*5)) {}

  bool operator<(Five_Year const& fy) const
  {
    return year < fy.year;
  }
  
  string interval() const
  {
    return to_string(year) + "-" + to_string(year+4);
  }
  
private:
  int year;
};