#include 

using namespace std;

int main()
{
    int p, d, n, zbir = 0;
    cin >> p >> d >> n;
    for (int i = 1; i < n; i++)
    {
        if (i == 1)
        {
            zbir += p;
        }
        else {p += d;
        zbir += p;
        }
    }
    cout << zbir << endl;
    return 0;
}