Не могу найти понять почему выдает ошибку

#include <iostream>
#include <math.h>
using namespace std;
 
class book
{
protected:
    int year;
book(int iyear):year(iyear){}
void set(int iyear)
{year=iyear;}
int  get()
{return year;}
int  let_knige()
{return 2016-year; }
};
class book_magazin:public book
{
private: double cost;
public:
book_magazin(int iyear,double c) :book(iyear), cost(c){}
int cena()
{
//if (let_knige()>5)
return cost*0.8;
}
};
int main()
{
string c,d;
cout << "vedite im9 avtora: ";
    getline(cin,c);
 
    cout << "Avtor: " << c << "!!! \n";
    cout << "nazvanie knigi : ";
    getline(cin,d);
 
    cout << "vashe naimenovanie: " << d << "!!! \n";
 
       int a;
       double b;
      cout <<  "God izdani9 knigi="; cin >> a;
      cout <<  "cena knigi="; cin >> b;
book_magazin h(c,d,a,b);
  cout  << "Let knige=" << h.let_knige() << endl;
    cout << "Cena knigi=" << h.cena() << endl;
system("pause");
        return 0;
}

class book
{
protected:
    int year;
    book(int iyear) :year(iyear) {}
    void set(int iyear)
    {
        year = iyear;
    }
    int  get()
    {
        return year;
    }
    int  let_knige()
    {
        return 2016 - year;
    }
};
class book_magazin :public book
{
private: double cost;
public:
    int let_knige() { book::let_knige(); }
    book_magazin(int iyear, double c) :book(iyear), cost(c) {}
    int cena()
    {
        if (let_knige()>5)
        return cost*0.8;
    }
};

class book {
protected:
    int year;
    string auth_name;
    string title;
 
    book(int iyear, string iauth_name, string ititle) : 
        year(iyear), title(ititle), auth_name(iauth_name) {}
 
    void set(int iyear) { year = iyear; }
    int  get() { return year; }
    int  let_knige() { return 2016 - year; }
 
};
 
class book_magazin :public book {
private:
    double cost;
 
public:
    int let_knige() { return book::let_knige(); }
 
    book_magazin(int iyear, string iauth_name, string ititle, double price) :
        book(iyear, iauth_name, ititle), cost(price) {}
 
    int cena() {
        if (let_knige()>5)
        return cost*0.8;
    }
};
int main()
{
    string auth_name, title;
    cout << "vedite im9 avtora: ";
    getline(cin, auth_name);
 
    cout << "Avtor: " << auth_name << "!!! \n";
    cout << "nazvanie knigi : ";
    getline(cin, title);
 
    cout << "vashe naimenovanie: " << title << "!!! \n";
 
    int year;
    double price;
    cout << "God izdani9 knigi="; cin >> year;
    cout << "cena knigi="; cin >> price;
 
    book_magazin h(year, auth_name, title, price);
 
    cout << "Let knige=" << h.let_knige() << endl;
    cout << "Cena knigi=" << h.cena() << endl;
    system("pause");
    return 0;
}

int cena() {
        if (let_knige()>5)
        return cost*0.8;
    }

double cena() {
        if (let_knige() > 5)
            return cost*0.8;
        else
            return cost;
    }