#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using ll =  long long ;
using P = pair<ll,ll> ;
const int INF = 1e9;
const int MOD = 1000000007;

int main(){
    ll n,k;
    cin >> n >> k;
    if(k==0){
        cout << 1 << endl;
        return 0;
    }else{
        cout << 1 + n*k - k*(k-1)/2 << endl;
    }
    return 0;
}