#include <iostream>
using namespace std;

int &f(int n = -1)
{
    static int c;
    if (n >= 0) {
        c = n * 10;
    }
    return c;
}

int main()
{
    cout << f(1) << endl;
    cout << f() << endl;
    f() = 2;
    cout << f() << endl;
    f(3);
    cout << f() << endl;
    return 0;
}