Why is it giving the error : "abc is inaccessible"?

#include <iostream>
using namespace std;
class B;
class A
{
    int a = 10;
    int b = 20;
public:
    friend void A::display(B&a);
    void display(B & c);
};
class B
{
    int abc = 20;

public:
    friend void A::display(B&a);
};
void A::display(B & c)
{
    cout << c.abc;
}
int main()
{
    B b;
    A a;
    a.display(b);
    return 0;
}

it is giving the error on the 21th line that "abc" is inaccessible. Although it is a friend function and have access to B class' private data members