// olimpudkiB.cpp: определяет точку входа для консольного приложения.
//

//#include "stdafx.h"
#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <algorithm>
#include <set>

using namespace std;



vector < pair<int, int> > prufer_decode(const vector<int> & prufer_code) {
    int n = (int)prufer_code.size() + 2;
    vector<int> degree(n, 1);
    for (int i = 0; i < n - 2; ++i)
        ++degree[prufer_code[i]];

    set<int> leaves;
    for (int i = 0; i < n; ++i)
        if (degree[i] == 1)
            leaves.insert(i);

    vector < pair<int, int> > result;
    for (int i = 0; i < n - 2; ++i) {
        int leaf = *leaves.begin();
        leaves.erase(leaves.begin());

        int v = prufer_code[i];
        result.push_back(make_pair(leaf, v));
        if (--degree[v] == 1)
            leaves.insert(v);
    }
    result.push_back(make_pair(*leaves.begin(), *--leaves.end()));
    return result;
}

bool comp(const pair<int, int> &a, const pair<int, int> &b)
{
    if (a.first < b.first)
    {
        return true;
    }
    else if (a.first == b.first)
    {
        if (a.second < b.second)
        {
            return true;
        }
        else
        {
            return false;
        }
    }
    else
    {
        return false;
    }
}
int main()
{

    long long a;
    vector<int> prufer_code;
    while (cin >> a)
    {
        if (!a)
        {
            break;
        }
        prufer_code.push_back(a - 1);
    }
    vector < pair<int, int> > edges = prufer_decode(prufer_code);
    for (int i = 0; i < edges.size(); ++i)
    {
        if (edges[i].first > edges[i].second)
        {
            swap(edges[i].first, edges[i].second);
        }
    }
    sort(edges.begin(), edges.end(), comp);
    for (int i = 0; i < edges.size(); ++i)
    {
        cout << edges[i].first + 1 << ' ' << edges[i].second + 1 << endl;
    }
    //system("pause");
    return 0;
}