#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <array>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
#include <iterator>
#include <memory>
#include <regex>
using namespace std;

long long sum(int a)
{
    return a * (a + 1LL) / 2;
}

int main()
{
    int n, k;
    cin >> n >> k;

    long long ans = 1 + sum(n) - sum(n-k);
    cout << ans << endl;

    return 0;
}