다음 Java 프로그램의 실행 결과를 쓰시오.

class Counter {
    private static final Counter INSTANCE = new Counter();
    private int value = 0;

    private Counter() {}
    static Counter getInstance() { return INSTANCE; }
    void add() { value++; }
    int getValue() { return value; }
}

public class Main {
    public static void main(String[] args) {
        Counter a = Counter.getInstance();
        Counter b = Counter.getInstance();
        a.add(); b.add(); a.add(); b.add();
        System.out.print(a.getValue());
    }
}