using System;

public class Hello
{
    static void Main()
    {
        string[] line = Console.ReadLine().Trim().Split(' ');
        var n = long.Parse(line[0]);
        var k = int.Parse(line[1]);
        var max = getSum(1, n);
        var min = getSum(1, n - k) - getSum(n -k + 1L, n);
        var ans = (max - min) / 2 + 1L;
        Console.WriteLine(ans);
    }
    static long getSum(long a, long b) => (b + a) * (b - a + 1) / 2L;
}