Problem:
Score : 200 points

Problem Statement

You have N apples, called Apple 1, Apple 2, Apple 3, ..., Apple N. The flavor of Apple i is L+i-1, which can be negative.

You can make an apple pie using one or more of the apples. The flavor of the apple pie will be the sum of the flavors of the apples used.

You planned to make an apple pie using all of the apples, but being hungry tempts you to eat one of them, which can no longer be used to make the apple pie.

You want to make an apple pie that is as similar as possible to the one that you planned to make. Thus, you will choose the apple to eat so that the flavor of the apple pie made of the remaining N-1 apples will have the smallest possible absolute difference from the flavor of the apple pie made of all the N apples.

Find the flavor of the apple pie made of the remaining N-1 apples when you choose the apple to eat as above.

We can prove that this value is uniquely determined.

Constraints

2 \leq N \leq 200

-100 \leq L \leq 100

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N L

Output

Find the flavor of the apple pie made of the remaining N-1 apples when you optimally choose the apple to eat.

Sample Input 1

5 2

Sample Output 1

18

The flavors of Apple 1, 2, 3, 4, and 5 are 2, 3, 4, 5, and 6, respectively. The optimal choice is to eat Apple 1, so the answer is 3+4+5+6=18.

Sample Input 2

3 -1

Sample Output 2

0

The flavors of Apple 1, 2, and 3 are -1, 0, and 1, respectively. The optimal choice is to eat Apple 2, so the answer is (-1)+1=0.

Sample Input 3

30 -50

Sample Output 3

-1044

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <vector>
#include <map>
#include <set>
#include <cmath>
#include <iostream>
#include <algorithm>
#include <queue>
#include <iomanip>
#include <list>
#include <string>

typedef char                SINT8;
typedef unsigned char       UINT8;
typedef short               SINT16;
typedef unsigned short      UINT16;
typedef int                 SINT32;
typedef unsigned int        UINT32;
typedef long long           SINT64;
typedef unsigned long long  UINT64;
typedef double              DOUBLE;

#define MAX(a,b) ((a)>(b)?(a):(b))
#define MIN(a,b) ((a)<(b)?(a):(b))
#define ABS(a) ((a)>(0)?(a):-(a))
#define rep(i,a,b) for(int (i)=int(a);(i)<int(b);(i)++)
#define rrep(i,a,b) for(int (i)=int(a);(i)>=int(b);(i)--)

#define put(a) cout << (a) << endl
#define puts(a) cout << (a) << " "
#define pute(a) cout << endl


#define INF 1000000001
#define MOD 1000000007
#define INF64 1000000000000000001

#define F first
#define S second

#define Pii  pair<SINT32,SINT32>
#define Pll  pair<SINT64,SINT64>
#define Piii pair<SINT32,pair<SINT32,SINT32>>
#define Plll pair<SINT64,pair<SINT64,SINT64>>

#define Vll(a,b,c)    vector<vector<SINT64>> (a)((b),vector<SINT64>((c))
#define Vlll(a,b,c,d) vector<vector<vector<SINT64>>> (a)((b),vector<vector<SINT64>>((c),vector<SINT64>((d)))


using namespace std;

int main() {

	SINT64 N;	cin >> N;
	SINT64 L;	cin >> L;
	SINT64 sum = 0;
	
	vector<SINT64> data(N);
	
	rep(i,0,N) {
		data[i] = i + L;
		sum += data[i];
	}

	SINT64 sa = INF;
	SINT64 ans = sum;
	rep(i,0,N) {
		SINT64 bsum = 0;
		rep(j,0,N) {
			if (i!=j) {
				bsum += data[j];
			}
		}
		if(sa > sum-bsum) {
			sa = ABS(sum-bsum);
			ans = bsum;
		}

	}

	put(ans);
	return 0;
}


//	vector<vector<SINT64>> data(N,vector<SINT32>(3));										//2次元配列
//	vector<vector<vector<SINT64>>> data(N,vector<vector<SINT64>>(3,vector<SINT64>(3)));		//3次元配列

//	Vll(data,N,N);		//2次元配列
//	Vlll(data,N,N,N);	//3次元配列

//	sort(data.begin(),data.end());
//	sort(data.begin(),data.end(),std::greater<SINT64>());
//	__gcd(A,B);


/* 複数条件ソート
bool sortcompare(Pll A, Pll B) {
    if(A.F == B.F){
        return A.S > B.S;
    } else {
        return A.F < B.F;
    }
}

sort(data.begin(),data.end(),sortcompare);
*/


//	data.emplace_back(BUF);			//後ろに追加

//  data.erase(std::unique(data.begin(), data.end()), data.end());	//ソート後に使用 同じ値を消す

//	data.insert(data.begin() + X, 0);	//X番目の要素に0を挿入


//	隣接リスト
//	vector<vector<SINT64>> data(N);
//	data[ A ].emplace_back( B );


/*
	vector<Pll> data(N);
	rep(i,0,N) {
		cin >> data[i].F;
		cin >> data[i].S;
	}
	sort(data.begin(),data.end());
*/

/*
	vector<Plll> data(N);
	rep(i,0,N) {
		cin >> data[i].F;
		cin >> data[i].S.F;
		cin >> data[i].S.S;
	}
	sort(data.begin(),data.end());
*/


// posi = lower_bound(data.begin(),data.end(), X) - data.begin();				// X以上を探す
// posi = lower_bound(data.begin(),data.end(),make_pair(X,0)) - data.begin();	//pair


/* 文字列回転
	string N;
	cin >> N;
	N = N[N.length()-1] + N.substr(0,N.length()-1);


	s = to_string(i);	//ストリング変換
*/
/* 文字列合成
	string N,M;
	cin >> N >> M;
	SINT64 ans = 0;
	ans = stoi(N+M);
*/

/*
//ワーシャルフロイド
vector<vector<SINT32>> dist(N,vector<SINT32>(N));
rep(i,0,N) {
	rep(j,0,N) {
		if (i != j) {
			dist[i][j] = INF;
		}
	}
}
rep(k,0,N) {
	rep(i,0,N) {
		rep(j,0,N) {
			dist[i][j] = MIN(dist[i][j], dist[i][k]+dist[k][j]);
		}
	}
}
*/

/*  優先度付きキュー
	priority_queue<SINT64, vector<SINT64>, greater<SINT64>> data;	//小さいほうから取り出せる
	priority_queue<SINT64, vector<SINT64>> data;					//大きいほうから取り出せる

	data.push(X);	//X を挿入
	data.top();		//先頭データ読み
	data.pop();		//先頭データ削除
*/





/* SET コンテナ
	set<SINT64> data;
	data.insert(X);				//X を挿入
	data.erase(data.begin());	//先頭を削除
	data.erase(--data.end());	//末尾を削除
	*data.begin();				//先頭要素にアクセス
	*data.rbegin();				//末尾要素にアクセス

	//全表示
    set<SINT64>::iterator it; //イテレータを用意
    for(it = data.begin(); it != data.end(); it++) {
        cout << *it << " ";
    }
	cout << endl;

	//N番目を一部表示
	set<SINT64>::iterator it; //  イテレータを用意
	it = data.begin();
	rep (i,0,N) {
		it++;
	}
	cout << *it << endl;
*/

/* map
	map<string,SINT32> mp;
	SINT32 N = 0;
	SINT32 mx = 0;

	cin >> N;
	for (SINT32 i = 0; i < N; i++) {
		string s;
		cin >> s;
		mp[s]++;
	}
	for(auto it=mp.begin();it!=mp.end();it++) {
		mx=max(mx,it->second);
	}
*/

/*
	//順列全表示
	//sortしてからでないと全列挙にならない
	sort(data.begin(),data.end());
	do {
		cout << buf << endl;
		rep(i,0,R) {
			cout << data[i] << " ";
		}
		cout << endl;

    } while (next_permutation(data.begin(),data.end()));
*/

//  桁指定表示
//  ans = ans * M_PI;
//	cout << setprecision(15) << ans << endl;


// 逆元　コンビネーション
/*
SINT64 modpow(SINT64 a, SINT64 p) {
	if (p == 0) return 1;
	if (p % 2 == 0) {
		//pが偶数の時
		SINT64 halfP = p / 2;
		SINT64 half = modpow(a, halfP);
		//a^(p/2) をhalfとして、half*halfを計算
		return half * half % MOD;
	} else {
		//pが奇数の時は、偶数にするために1減らす
		return a * modpow(a, p - 1) % MOD;
	}
}

SINT64 calcComb(SINT64 a, SINT64 b) {
	SINT64 Mul = 1;
	SINT64 Div = 1;
	SINT64 ans = 0;

	if (b > a - b) {
		return calcComb(a, a - b);
	}
 
	rep(i,0,b) {
		Mul *= (a - i);
		Div *= (i + 1);
		Mul %= MOD;
		Div %= MOD;
	}
	ans = Mul * modpow(Div, MOD - 2) % MOD;
	return ans;
}
*/