What will be the output of the program?

#include <iostream>
using namespace std;
class A { public: virtual void f() { cout << "A"; } virtual ~A() {} };
class B : public A { public: void f() override { cout << "B"; } };
int main() {
    B b;
    A &r = b;
    r.f();
    cout << endl;
    return 0;
}