#include<bits/stdc++.h>
#define REP(i,n) for(int i=0;i<(n);i++)
#define ALL(v) (v).begin(),(v).end()
#define SZ(x) ((int)(x).size())
#define int long long
using namespace std;

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