package chelper;

import java.util.ArrayList;
import java.util.List;

import io.InputReader;
import io.OutputWriter;
import misc.SimpleSavingChelperSolution;


public class A extends SimpleSavingChelperSolution {

  public void solve(int testNumber, InputReader in, OutputWriter out) {
    wrapSolve(testNumber, in, out);
  }

  @Override
  public void solve(int testNumber) {
    int n = in.nextInt();

    int[] l = new int[n];
    int[] r = new int[n];

    for (int i = 0; i < n; i++) {
      l[i] = in.nextInt() - 1;
      r[i] = in.nextInt() - 1;
    }

    boolean[] visited = new boolean[n];

    List<int[]> things = new ArrayList<>();

    for (int i = 0; i < n; i++) {
      if (visited[i]) {
        continue;
      }

      int[] t = dfs(i, n, l, r, visited);
      things.add(t);
    }

    for (int i = 0; i < things.size() - 1; i++) {
      int[] t1 = things.get(i);
      int[] t2 = things.get(i + 1);

      r[t1[1]] = t2[0];
      l[t2[0]] = t1[1];
    }

    for (int i = 0; i < n; i++) {
      out.println((l[i] + 1) + " " + (r[i] + 1));
    }
  }

  int[] dfs(int v, int n, int[] l, int[] r, boolean[] visited) {
    visited[v] = true;

    int leftmost = v;
    int rightmost = v;

    if (l[v] != -1 && !visited[l[v]]) {
      int[] t = dfs(l[v], n, l, r, visited);
      leftmost = t[0];
    }

    if (r[v] != -1 && !visited[r[v]]) {
      int[] t = dfs(r[v], n, l, r, visited);
      rightmost = t[1];
    }

    return new int[]{leftmost, rightmost};
  }
}