Find the Pivot Integer
<p>Given a positive integer <code>n</code>, find the <strong>pivot integer</strong> <code>x</code> such that:</p>

<ul>
	<li>The sum of all elements between <code>1</code> and <code>x</code> inclusively equals the sum of all elements between <code>x</code> and <code>n</code> inclusively.</li>
</ul>

<p>Return <em>the pivot integer </em><code>x</code>. If no such integer exists, return <code>-1</code>. It is guaranteed that there will be at most one pivot index for the given input.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> n = 8
<strong>Output:</strong> 6
<strong>Explanation:</strong> 6 is the pivot integer since: 1 + 2 + 3 + 4 + 5 + 6 = 6 + 7 + 8 = 21.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> n = 1
<strong>Output:</strong> 1
<strong>Explanation:</strong> 1 is the pivot integer since: 1 = 1.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> n = 4
<strong>Output:</strong> -1
<strong>Explanation:</strong> It can be proved that no such integer exist.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= n &lt;= 1000</code></li>
</ul>
Math; Prefix Sum
class Solution {
    /**
     * @param Integer $n
     * @return Integer
     */
    function pivotInteger($n) {
        $sum = ($n * ($n + 1)) / 2;
        $pre = 0;
        for ($i = 1; $i <= $n; $i++) {
            if ($pre + $i === $sum - $pre) {
                return $i;
            }
            $pre += $i;
        }
        return -1;
    }
}
