#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
using ll = long long;

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

    ll s = 0;
    for (int i = 0; i < k; i++) {
        s += n - i;
    }

    cout << s + 1 << endl;

    return 0;
}