#include <bits/stdc++.h>
using namespace std;
pair<int, int> nextInt(const string &s, int ind) {
  int ret = 0;
  while (ind < s.length() && s[ind] >= '0' && s[ind] <= '9') {
    ret = ret * 10 + ((int)s[ind] - 48);
    ind++;
  }
  return make_pair(ret, ind);
}
int main() {
  int counti = 0;
  int n;
  int w[1024];
  vector<vector<int> > input;
  for (int i = 0; i < 1024; i++) {
    w[i] = 0;
  }
  cin >> n;
  counti = n;
  input.resize(n + 10);
  for (int i = 1; i <= n; i++) {
    input[i].push_back(-1);
    string in;
    cin >> in;
    if (nextInt(in, 0).first == n - 1) {
      w[i] = 1;
    } else {
      int ind = 0;
      while (in[ind]) {
        if (in[ind] >= '0' && in[ind] <= '9') {
          pair<int, int> ret = nextInt(in, ind);
          ind = ret.second;
          input[i].push_back(ret.first);
        } else if (in[ind] == '-') {
          input[i].push_back(-1);
          ind++;
          counti++;
        } else
          ind++;
      }
    }
  }
  int weight = 1;
  vector<pair<int, int> > ans;
  while (weight < n) {
    for (int i = 1; i <= n; i++) {
      for (int j = 1; j < input[i].size(); j++)
        if (input[i][j - 1] == -1 && input[i][j] == weight)
          for (j = j + 1; j < input[i].size() && input[i][j] != -1; j++)
            if (w[input[i][j]] == weight) {
              ans.push_back(make_pair(i, input[i][j]));
              if (w[i] == 0)
                w[i] = weight + 1;
              else
                w[i] += weight;
            }
    }
    weight++;
  }
  if (n == 2) {
    cout << "1" << endl;
    cout << "1 2";
    return 0;
  }
  if (ans.size() != n - 1 || counti != (n - 1) * 2) {
    cout << -1;
    return 0;
  }
  cout << ans.size() << endl;
  for (int i = 0; i < ans.size(); i++)
    cout << ans[i].first << " " << ans[i].second << endl;
  return 0;
}
