#include <bits/stdc++.h>
using namespace std;
//#define MOD 1000000007
#define MOD 998244353
#define INF 100000000000000009
typedef long long ll;
#define REP(i,n) for(int i=0;i<(n);++i)
#define OREP(i,n) for(int i=1;i<=(n);++i)
#define ZREP(i,n) for(int i=1;i<(n);++i)
#define YES(s) s?cout << "YES" << endl:cout << "NO" << endl
#define Yes(s) s?cout << "Yes" << endl:cout << "No" << endl
#define out(s,t,u) s?cout << t << endl:cout << u << endl
#define int ll
#define Endl endl


signed main(){
    int N,K;
    cin >> N >> K;
    int M=N*(N+1)/2;
    int R=M;
    R-=(N-K)*(N-K+1);
    cout << (M+R+2)/2 << endl;
    
    return 0;
}