A10821 | News About Credit

The first line contains the positive integer $n$ ( $2<=n<=100$ ) — the number of students.

The second line contains the sequence $a_{1},a_{2},...,a_{n}$ ( $0<=a_{i}<=100$ ), where $a_{i}$ equals to the maximum number of messages which can the $i$ -th student agree to send. Consider that Polycarp always has the number $1$ .

Print -1 to the first line if it is impossible to inform all students about credit.

Otherwise, in the first line print the integer $k$ — the number of messages which will be sent. In each of the next $k$ lines print two distinct integers $f$ and $t$ , meaning that the student number $f$ sent the message with news to the student number $t$ . All messages should be printed in chronological order. It means that the student, who is sending the message, must already know this news. It is assumed that students can receive repeated messages with news of the credit.

If there are several answers, it is acceptable to print any of them.

In the first test Polycarp (the student number $1$ ) can send the message to the student number $2$ , who after that can send the message to students number $3$ and $4$ . Thus, all students knew about the credit.