CF492B.Vanya and Lanterns

Vanya walks late at night along a straight street of length l, lit by n lanterns. Consider the coordinate system with the beginning of the street corresponding to the point 0, and its end corresponding to the point l. Then the i-th lantern is at the point a__i. The lantern lights all points of the street that are at the distance of at most d from it, where d is some positive number, common for all lanterns.

Vanya wonders: what is the minimum light radius d should the lanterns have to light the whole street?

瓦尼亚深夜沿着一条长度为 $l$ 的笔直街道行走，街道上共有 $n$ 盏路灯。我们建立一个坐标系，使街道起点对应坐标 $0$，终点对应坐标 $l$。第 $i$ 盏路灯位于坐标 $a_i$ 处。每盏路灯的照明范围是其周围距离不超过 $d$ 的所有点，其中 $d$ 是一个正数，且对所有路灯均相同。

瓦尼亚想知道：为了照亮整条街道，路灯所需的最小照明半径 $d$ 是多少？

The first line contains two integers n, l (1 ≤ n ≤ 1000, 1 ≤ l ≤ 109) — the number of lanterns and the length of the street respectively.

The next line contains n integers a__i (0 ≤ a__i ≤ l). Multiple lanterns can be located at the same point. The lanterns may be located at the ends of the street.

第一行包含两个整数 $n$、$l$（$1 \leq n \leq 1000$，$1 \leq l \leq 10^9$）—— 分别表示灯笼的数量和街道的长度。

下一行包含 $n$ 个整数 $a_i$（$0 \leq a_i \leq l$）。多个灯笼可以位于同一位置。灯笼可以位于街道的端点处。

Print the minimum light radius d, needed to light the whole street. The answer will be considered correct if its absolute or relative error doesn't exceed 10 - 9.

输出照亮整条街道所需的最小灯光半径 $d$。若答案的绝对或相对误差不超过 $10^{-9}$，则视为正确。

Consider the second sample. At d = 2 the first lantern will light the segment [0, 4] of the street, and the second lantern will light segment [3, 5]. Thus, the whole street will be lit.

考虑第二个样例。当 $d = 2$ 时，第一盏灯笼将照亮街道上的区间 $[0,\,4]$，第二盏灯笼将照亮区间 $[3,\,5]$。因此，整条街道都将被照亮。