#include <cstdio>
#include <cstdlib>
#include <cstddef>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>
#include <iostream>
#include <iomanip>
#define L64 long long
#define MOD (998244353LL)

int counter = 0;

int main(void)
{
  L64 a, b;
  std::cin >> a >> b;
  L64 ret = ((a + (a - b + 1)) * b) / 2 + 1;
  std::cout << ret << std::endl;
}