#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define int long long
using namespace std;
typedef pair<int, int> P;
const int mod = 1000000007;
const int INF = 1e18;

signed main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    int n, k;
    cin >> n >> k;
    int MAX = n * (n + 1) / 2;
    int MIN = MAX - k * (n - k + 1 + n);
    cout << (MAX - MIN) / 2 + 1 << endl;
}