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 PHP (7.4.4) (judge verdict: Runtime Error)

<?php
// D
define("MOD", 10 ** 9 + 7);
fscanf(STDIN, '%d %d', $n, $k);
$ans = 0;
for ($i = $k; $i <= $n + 1; $i++) {
    $min = ($i - 1)* $i / 2;
    $max = (2 * $n - $i + 1) * $i / 2;
    $ans = modAdd($ans, $max - $min + 1));
}
echo $ans;

// 足し算
function modAdd($x, $y)
{
    return ($x + $y) % MOD;
}

// 引き算
function modSub($x, $y)
{
    return ($x + MOD - $y) % MOD;
}

// 掛け算
function modMul($x, $y)
{
    return ($x * $y) % MOD;
}

// 割り算
function modDiv($x, $y)
{
    return modMul($x, modPow($y, MOD - 2));
}

// 累乗（繰り返し二乗法）
function modPow($n, $x)
{
    if ($x === 0) return 1;
    $res = (modPow($n, $x >> 1) ** 2) % MOD;
    if ($x % 2 === 1) $res = modMul($res, $n);
    return $res;
}

// 階乗
function modFac($n)
{
    if ($n === 0) return 1;
    return modMul($n, modFac($n - 1));
}

// 順列
function nPr($n, $r)
{
    if ($r === 0) return 1;
    return modDiv(modFac($n), modFac($n - $r));
}

// 組み合わせ
function nCr($n, $r)
{
    $r = min($r, $n - $r);
    if ($r === 0) return 1;
    return modDiv(nPr($n, $r), modFac($r));
}

// nCr 前処理　配列を返す
function nCr2($max)
{
    $pascal[0][0] = 1;
    for ($j = 1; $j <= $max; $j++) {
        for ($i = 0; $i <= $j; $i++) {
            $pascal[$j][$i] = (($pascal[$j - 1][$i - 1] ?? 0) + ($pascal[$j - 1][$i] ?? 0)) % MOD;
        }
    }
    return $pascal;
}