#include <bits/stdc++.h>
using namespace std;
using ll = long long;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    ll N, K, M;
    cin >> N >> K;
    M = N - K;
    cout << 1LL + N * (N + 1) / 2LL - M * (M + 1) / 2LL << endl;

    return 0;
}