#include <iostream>


int main()
{
    int heads;
    int time;

    int endHeads = 0;

    std::cin >> heads >> time;

    int a = 0; // factor

    for (int i = 0; i < time; i++)
    {
        a = a + i * 2;
        endHeads = heads - a + (i * 3) * 2;

    }

    std::cout << endHeads;

    return 0;
}