CF899A.Splitting in Teams

There were n groups of students which came to write a training contest. A group is either one person who can write the contest with anyone else, or two people who want to write the contest in the same team.

The coach decided to form teams of exactly three people for this training. Determine the maximum number of teams of three people he can form. It is possible that he can't use all groups to form teams. For groups of two, either both students should write the contest, or both should not. If two students from a group of two will write the contest, they should be in the same team.

共有 $n$ 个学生小组前来参加一场训练赛。每个小组要么由一名学生组成（该学生可与任意其他学生组队），要么由两名学生组成（这两人必须在同一支队伍中参赛）。

教练决定为本次训练赛组建恰好由三人组成的队伍。请确定他最多能组成多少支三人队伍。注意，他可能无法使用全部小组来组队。对于二人小组，其中的两名学生必须同时参赛或同时不参赛；若他们参赛，则必须被分在同一支三人队伍中。

The first line contains single integer n (2 ≤ n ≤ 2·105) — the number of groups.

The second line contains a sequence of integers _a_1, _a_2, ..., a__n (1 ≤ a__i ≤ 2), where a__i is the number of people in group i.

第一行包含一个整数 $n$（$2 \leq n \leq 2\cdot10^5$）—— 组的数量。

第二行包含一个整数序列 $a_1, a_2, ..., a_n$（$1 \leq a_i \leq 2$），其中 $a_i$ 表示第 $i$ 组的人数。

Print the maximum number of teams of three people the coach can form.

输出教练最多能组成的三人队伍数量。

In the first example the coach can form one team. For example, he can take students from the first, second and fourth groups.

In the second example he can't make a single team.

In the third example the coach can form three teams. For example, he can do this in the following way:

• The first group (of two people) and the seventh group (of one person),
• The second group (of two people) and the sixth group (of one person),
• The third group (of two people) and the fourth group (of one person).

在第一个例子中，教练可以组成一支队伍。例如，他可以从第一组、第二组和第四组中挑选学生。

在第二个例子中，教练无法组成任何一支队伍。

在第三个例子中，教练可以组成三支队伍。例如，他可以按如下方式操作：

• 第一组（两人）和第七组（一人），
• 第二组（两人）和第六组（一人），
• 第三组（两人）和第四组（一人）。