**5 人解决**，12 人已尝试。

**6 份提交通过**，共有 40 份提交。

**8.2** EMB 奖励。

**单点时限: **2.0 sec

**内存限制: **256 MB

There are $n$ towns with their own airports in the country X. We know the maximal capacities of the airports - the airport in the town $M_i$ can have at most $d_i$ connections with other towns. The task is to design the net of air connections among the towns in such a way that the town $M_i$ has exactly $d_i$ connections with other towns. We assume that connections are two-way and that each pair of towns has at most one connection between them.

Write a program that:

- reads the number of towns $n$ and the numbers $d_i$,
- designs the net of air connections in such a way that for every $i$, $1 \le i \le n$, the town $M_i$ has exactly $d_i$ connections with other towns,
- writes the list of all connections.

We assume that for the given data a solution exists. If there exists more than one solution the program should find only one.

In the first line there is written one integer $n$, $3 \le n \le 500$, which is the number of towns.

In the following $n$ lines there are written positive integers $d_i$ (one integer in each line).

Your program should write all the connections of the created net. The description of each connection consists of two positive integers separated by a single space. These integers are the numbers of two connected towns. Each description should be placed in a separate line. The numbers of towns in a line can be written in an arbitrary order. Similarly, the order of connections is not important.

Input

6 2 3 2 4 1 2

Output

5 4 4 2 1 2 2 3 6 3 4 6 4 1

**5 人解决**，12 人已尝试。

**6 份提交通过**，共有 40 份提交。

**8.2** EMB 奖励。

题目标签