CF604B.More Cowbell

Kevin Sun wants to move his precious collection of n cowbells from Naperthrill to Exeter, where there is actually grass instead of corn. Before moving, he must pack his cowbells into k boxes of a fixed size. In order to keep his collection safe during transportation, he won't place more than two cowbells into a single box. Since Kevin wishes to minimize expenses, he is curious about the smallest size box he can use to pack his entire collection.

Kevin is a meticulous cowbell collector and knows that the size of his i-th (1 ≤ i ≤ n) cowbell is an integer s__i. In fact, he keeps his cowbells sorted by size, so s__i - 1 ≤ s__i for any i > 1. Also an expert packer, Kevin can fit one or two cowbells into a box of size s if and only if the sum of their sizes does not exceed s. Given this information, help Kevin determine the smallest s for which it is possible to put all of his cowbells into k boxes of size s.

凯文·孙想要将他珍贵的 $n$ 个牛铃收藏品从纳珀斯里尔运送到埃克塞特——那里长着青草，而不是玉米。在运输之前，他必须将这些牛铃装进 $k$ 个尺寸固定的盒子中。为了在运输过程中确保收藏品的安全，他不会在单个盒子中放置超过两个牛铃。由于凯文希望尽量减少开支，他很好奇：能够装下全部牛铃的最小盒子尺寸是多少？

凯文是一位细致入微的牛铃收藏家，他知道第 $i$ 个（$1 \le i \le n$）牛铃的尺寸是一个整数 $s_i$。事实上，他将牛铃按尺寸升序排列，因此对任意 $i > 1$ 都有 $s_{i-1} \le s_i$。同时，作为一名包装专家，凯文仅当两个牛铃的尺寸之和不超过 $s$ 时，才能将它们（或一个牛铃）装入一个尺寸为 $s$ 的盒子中。已知上述信息，请帮助凯文确定最小的 $s$，使得他能将全部牛铃装入 $k$ 个尺寸为 $s$ 的盒子中。

The first line of the input contains two space-separated integers n and k (1 ≤ n ≤ 2·k ≤ 100 000), denoting the number of cowbells and the number of boxes, respectively.

The next line contains n space-separated integers _s_1, _s_2, ..., s__n (1 ≤ _s_1 ≤ _s_2 ≤ ... ≤ s__n ≤ 1 000 000), the sizes of Kevin's cowbells. It is guaranteed that the sizes s__i are given in non-decreasing order.

输入的第一行包含两个以空格分隔的整数 $n$ 和 $k$（$1 ≤ n ≤ 2·k ≤ 100\,000$），分别表示牛铃的数量和箱子的数量。

下一行包含 $n$ 个以空格分隔的整数 $s_1,\,s_2,\,...,\,s_n$（$1 ≤ s_1 ≤ s_2 ≤ ... ≤ s_n ≤ 1\,000\,000$），表示 Kevin 的牛铃的尺寸。保证所给尺寸 $s_i$ 是按非递减顺序排列的。

Print a single integer, the smallest s for which it is possible for Kevin to put all of his cowbells into k boxes of size s.

输出一个整数，即 Kevin 能将所有牛铃放入 $k$ 个容量为 $s$ 的盒子时，所能达到的最小的 $s$ 值。

In the first sample, Kevin must pack his two cowbells into the same box.

In the second sample, Kevin can pack together the following sets of cowbells: {2, 3}, {5} and {9}.

In the third sample, the optimal solution is {3, 5} and {7}.

在第一个样例中，Kevin 必须将他的两个牛铃装入同一个箱子中。

在第二个样例中，Kevin 可以将以下几组牛铃分别装箱：{2, 3}、{5} 和 {9}。

在第三个样例中，最优方案是 {3, 5} 和 {7}。