Problem:
Score : 600 points

Problem Statement

We have an integer sequence A of length N, where A_1 = X, A_{i+1} = A_i + D (1 \leq i < N ) holds.

Takahashi will take some (possibly all or none) of the elements in this sequence, and Aoki will take all of the others.

Let S and T be the sum of the numbers taken by Takahashi and Aoki, respectively. How many possible values of S - T are there?

Constraints

-10^8 \leq X, D \leq 10^8

1 \leq N \leq 2 \times 10^5

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N X D

Output

Print the number of possible values of S - T.

Sample Input 1

3 4 2

Sample Output 1

8

A is (4, 6, 8).

There are eight ways for (Takahashi, Aoki) to take the elements: ((), (4, 6, 8)), ((4), (6, 8)), ((6), (4, 8)), ((8), (4, 6))), ((4, 6), (8))), ((4, 8), (6))), ((6, 8), (4))), and ((4, 6, 8), ()).

The values of S - T in these ways are -18, -10, -6, -2, 2, 6, 10, and 18, respectively, so there are eight possible values of S - T.

Sample Input 2

2 3 -3

Sample Output 2

2

A is (3, 0). There are two possible values of S - T: -3 and 3.

Sample Input 3

100 14 20

Sample Output 3

49805

Solution in C# (Mono 4.6.2.0) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;
using System.Numerics;
using static System.Math;
using static System.Array;
using static AtCoder.Tool;
namespace AtCoder
{
    class AC
    {
        //const int MOD = 1000000007;
        const int MOD = 998244353;
        const int INF = int.MaxValue / 2;
        const long SINF = long.MaxValue / 2;
        const double EPS = 1e-8;
        static readonly int[] dI = { 0, 1, 0, -1, 1, -1, -1, 1 };
        static readonly int[] dJ = { 1, 0, -1, 0, 1, 1, -1, -1 };
        //static List<List<int>> G = new List<List<int>>();
        //static List<List<Edge>> G = new List<List<Edge>>();
        //static List<Edge> E = new List<Edge>();
        static void Main(string[] args)
        {
            //var sw = new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false };
            //Console.SetOut(sw);
            var cin = new IO_ShortCut();

            long N, X, D;
            cin.Input(out N, out X, out D);
            if (D == 0) { cin.OutL(X == 0 ? 1 : N + 1); return; }
            if (D < 0) { X = -X;D = -D; }
            long ans = 0;
            Func<long, long, long> s = (a, b) => -a * (a - 1) / 2 + b * (b + 1) / 2;
            var rec = new Dictionary<int, List<long>>();
            for (long i = 0; i <= N; i++)
            {
                long rem = ((i * Abs(X)) % D);
                var r = (int)rem;
                if (!rec.ContainsKey(r)) rec[r] = new List<long>();
                rec[r].Add(i);
            }
            foreach(var rs in rec)
            {
                long r = rs.Key;
                var lis = rs.Value;
                lis.Sort((x, y) =>
                {
                    long dif = (x * X - r) + s(0, x - 1) * D - (y * X - r) - s(0, y - 1) * D;
                    return dif == 0 ? 0 : (dif > 0 ? 1 : -1);
                });
                long mx = -(long)1e15;
                foreach(var p in lis)
                {
                    var k = p;
                    long st = (k * X - r) / D + s(0, k - 1);
                    long ed = (k * X - r) / D + s(N - k, N - 1);
                    long num = ed - st + 1;
                    long no = Max(0, mx - st + 1);

                    //cin.Out_Sep(st, ed, mx);
                    ans += Max(0, num - no);
                    mx = Max(mx, ed);
                    //cin.OutL(ans);
                }
            }
            cin.OutL(ans);


            //Console.Out.Flush();
        }
        
        struct Edge
        {
            public int from;

            public int to;
            public long dist;
            public Edge(int t, long c)
            {
                from = -1;
                to = t;
                dist = c;
            }
            public Edge(int f, int t, long c)
            {
                from = f;
                to = t;
                dist = c;
            }
        }
        
    }
    
    class IO_ShortCut
    {
        public IO_ShortCut() { }
        public string[] ReadSplit => Console.ReadLine().Split();
        public int[] ReadSplitInt => ConvertAll(Console.ReadLine().Split(), int.Parse);
        public long[] ReadSplitLong => ConvertAll(Console.ReadLine().Split(), long.Parse);
        public double[] ReadSplit_Double => ConvertAll(Console.ReadLine().Split(), double.Parse);
        public string Str => Console.ReadLine();
        public int Int => int.Parse(Console.ReadLine());
        public long Long => long.Parse(Console.ReadLine());
        public double Double => double.Parse(Console.ReadLine());
        public T Conv<T>(string input) => (T)Convert.ChangeType(input, typeof(T));
        public void Input<T>(out T a) => a = Conv<T>(Console.ReadLine());
        public void Input<T, U>(out T a, out U b)
        { var q = ReadSplit;a = Conv<T>(q[0]);b = Conv<U>(q[1]); }
        public void Input<T, U, V>(out T a, out U b, out V c)
        { var q = ReadSplit;a = Conv<T>(q[0]);b = Conv<U>(q[1]);c = Conv<V>(q[2]); }
        public void Input<T, U, V, W>(out T a, out U b, out V c, out W d)
        { var q = ReadSplit; a = Conv<T>(q[0]); b = Conv<U>(q[1]); c = Conv<V>(q[2]);d = Conv<W>(q[3]); }

        public void OutL(object s) => Console.WriteLine(s);
        public void Out_Sep<T>(IEnumerable<T> s) => Console.WriteLine(string.Join(" ", s));
        public void Out_Sep<T>(IEnumerable<T> s, string sep) => Console.WriteLine(string.Join($"{sep}", s));
        public void Out_Sep(params object[] s) => Console.WriteLine(string.Join(" ", s));
        public void Out_One(object s) => Console.Write($"{s} ");
        public void Out_One(object s, string sep) => Console.Write($"{s}{sep}");
    }
    public static class Tool
    {
        static public void Initialize<T>(ref T[] array, T initialvalue)
        {
            array = ConvertAll(array, x => initialvalue);
        }
        static public void Swap<T>(ref T a, ref T b)
        {
            T keep = a;
            a = b;
            b = keep;
        }
        static public void Display<T>(T[,] array2d, int n, int m)
        {
            for (var i = 0; i < n; i++)
            {
                for (var j = 0; j < m; j++)
                {
                    Console.Write($"{array2d[i, j]} ");
                }
                Console.WriteLine();
            }
        }
        static public long LPow(int a, int b) => (long)Pow(a, b);
    }
    class Modulo
    {
        private int M;
        private readonly long[] m_facs;
        public long Mul(long a, long b)
        {
            return ((a * b) % M);
        }
        public Modulo(long n, int m)
        {
            M = m;
            m_facs = new long[n + 1];
            m_facs[0] = 1;
            for (long i = 1; i <= n; ++i)
            {
                m_facs[i] = Mul(m_facs[i - 1], i);
            }
        }
        public long Fac(long n)
        {
            return m_facs[n];
        }
        public long Pow(long a, long m)
        {
            switch (m)
            {
                case 0:
                    return 1L;
                case 1:
                    return a;
                default:
                    long p1 = Pow(a, m / 2);
                    long p2 = Mul(p1, p1);
                    return ((m % 2) == 0) ? p2 : Mul(p2, a);
            }
        }
        public long Div(long a, long b)
        {
            return Mul(a, Pow(b, M - 2));
        }
        public long Ncr(long n, long r)
        {
            if (n < r) { return 0; }
            if (n == r) { return 1; }
            long res = Fac(n);
            res = Div(res, Fac(r));
            res = Div(res, Fac(n - r));
            return res;
        }
    }
}