Problem:
Score : 400 points

Problem Statement

We have N+1 integers: 10^{100}, 10^{100}+1, ..., 10^{100}+N.

We will choose K or more of these integers. Find the number of possible values of the sum of the chosen numbers, modulo (10^9+7).

Constraints

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

1 \leq K \leq N+1

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N K

Output

Print the number of possible values of the sum, modulo (10^9+7).

Sample Input 1

3 2

Sample Output 1

10

The sum can take 10 values, as follows:

(10^{100})+(10^{100}+1)=2\times 10^{100}+1

(10^{100})+(10^{100}+2)=2\times 10^{100}+2

(10^{100})+(10^{100}+3)=(10^{100}+1)+(10^{100}+2)=2\times 10^{100}+3

(10^{100}+1)+(10^{100}+3)=2\times 10^{100}+4

(10^{100}+2)+(10^{100}+3)=2\times 10^{100}+5

(10^{100})+(10^{100}+1)+(10^{100}+2)=3\times 10^{100}+3

(10^{100})+(10^{100}+1)+(10^{100}+3)=3\times 10^{100}+4

(10^{100})+(10^{100}+2)+(10^{100}+3)=3\times 10^{100}+5

(10^{100}+1)+(10^{100}+2)+(10^{100}+3)=3\times 10^{100}+6

(10^{100})+(10^{100}+1)+(10^{100}+2)+(10^{100}+3)=4\times 10^{100}+6

Sample Input 2

200000 200001

Sample Output 2

1

We must choose all of the integers, so the sum can take just 1 value.

Sample Input 3

141421 35623

Sample Output 3

220280457

Solution in C# (Mono-mcs 6.8.0.105) (judge verdict: Wrong Answer)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace cysharp_test
{
    class Program
    {
        static void Main (string[] args)
        {
            new Program ().solve ();
        }

        void solve ()
        {
            long[] NK = Console.ReadLine ().Split (' ').Select (long.Parse).ToArray ();
            long N = NK[0];
            long K = NK[1];
            long count = 0;
            for (long i = K; i <= N + 1; i++)
            {
                /*long max = (N - i + 1 + N) * i / 2;
                long min = (0 + i) + i / 2;
                count += max - min + 1;*/

                count += i * N - 2 * sum(i - 1) + 1;
            }

            Console.WriteLine (count);
            Console.ReadLine();
        }

        static long sum (long n){
            long sum = 0;
            for (int i = 1; i <= n;i++){
                sum += i;
            }
            return sum;
        }

    }

    class Vector2
    {
        public Vector2 (long x, long y)
        {
            this.x = x;
            this.y = y;
        }

        long x, y;

        public static Vector2 operator + (Vector2 a, Vector2 b)
        {
            return new Vector2 (a.x + b.x, a.y + b.y);
        }

        public static Vector2 operator - (Vector2 a, Vector2 b)
        {
            return new Vector2 (a.x - b.x, a.y - b.y);
        }

        public static Vector2 operator * (Vector2 a, long scala)
        {
            return new Vector2 (a.x * scala, a.y + scala);
        }
    }
}