CF1268B.Domino for Young

普及/提高-

通过率：0%

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题目描述

You are given a Young diagram.

Given diagram is a histogram with $n$ columns of lengths $a_1, a_2, \ldots, a_n$ ( $a_1 \geq a_2 \geq \ldots \geq a_n \geq 1$ ).

Young diagram for $a=[3,2,2,2,1]$ .Your goal is to find the largest number of non-overlapping dominos that you can draw inside of this histogram, a domino is a $1 \times 2$ or $2 \times 1$ rectangle.

输入格式

The first line of input contain one integer $n$ ( $1 \leq n \leq 300\,000$ ): the number of columns in the given histogram.

The next line of input contains $n$ integers $a_1, a_2, \ldots, a_n$ ( $1 \leq a_i \leq 300\,000, a_i \geq a_{i+1}$ ): the lengths of columns.

输出格式

Output one integer: the largest number of non-overlapping dominos that you can draw inside of the given Young diagram.

输入输出样例

输入#1
```
5
3 2 2 2 1
```
输出#1
```
4
```

说明/提示

Some of the possible solutions for the example:

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