#include <algorithm>
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define all(x) (x).begin(), (x).end()
#define ll long long
#define ld long double
#define INF 1000000000000000000
typedef pair<ll, ll> pll;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    ll N, K;
    cin >> N >> K;
    ll MAX = (1 + N) * N / 2,
       MIN = (1 + N - K) * (N - K) / 2 - (N - K + 1 + N) * K / 2;
    cout << (MAX - MIN) / 2 + 1 << endl;
    return 0;
}