e156 - 2

/* e156 - 2        */
/* AC (3ms, 304KB) */
#include <cstdio>

using namespace std;

class A;

unsigned int n;
A *arr[2];

class A
{
    public:
        virtual unsigned int Sum(unsigned int n)
        {
            return 0;
        }
};

class B: public A
{
    public:
        virtual unsigned int Sum(unsigned int n)
        {
            return arr[!!n] -> Sum(n - 1) + n;
        }
};

unsigned int Sum_Solution(unsigned int n)
{
    A a;
    B b;
    arr[0] = &a;
    arr[1] = &b;

    unsigned int value = arr[1] -> Sum(n);

    return value;
}

int main()
{
    scanf("%u", &n);
    printf("%u", Sum_Solution(n));
    return 0;
}