#define _USE_MATH_DEFINES
#include <iostream>
#include <string>
#include <cmath>
#include <vector>
#include <algorithm>
#include <set>



int main()
{
	int n, x = 1, y = 1;
	std::cin >> n;

	std::vector<int> vec(2 * n, -1);

	for (int t = 0; t < 2 * n - 1; t++)
	{
		int i, j;
		std::cin >> i;
		i--;

		if (vec[i] == -1)
		{
			if (x <= n)
				vec[i] = x++;
			else
				vec[i] = y++;
		}

		std::cout << vec[i] << " ";

		std::cin >> j;
		j--;
		if (vec[j] == -1)
		{
			if (y <= n && vec[i] != y)
				vec[j] = y++;

			else
				vec[j] = x++;
		}

		std::cout << vec[j] << " ";
	}

	return 0;



}