#include <bits/stdc++.h>
using namespace std;
template <typename T>
void join(vector<T>& arr, string sep) {
  bool first = true;
  for (auto t : arr) {
    if (first) {
      first = false;
      cout << t;
    } else {
      cout << sep << t;
    }
  }
  cout << "\n";
}
struct Euler {
  vector<vector<pair<long long, long long>>> g;
  long long n;
  vector<long long> path;
  long long edge_count;
  Euler(long long n) : n(n) {
    g.resize(n);
    edge_count = 0;
  }
  void addEdge(long long u, long long v) {
    long long w = edge_count++;
    g[u].push_back({v, w});
    g[v].push_back({u, w});
  }
  bool compute() {
    int first = 0;
    while (first < n && g[first].empty()) first++;
    if (first == n) return true;
    long long v1 = -1, v2 = -1;
    for (long long u = 0; u < (long long)(n); ++u) {
      if (g[u].size() & 1) {
        return false;
      }
    }
    if (v1 != -1) {
      assert(v2 != -1);
    }
    vector<long long> visited(edge_count);
    vector<long long> s = {first};
    while (!s.empty()) {
      long long u = s.back();
      long long v;
      while (true) {
        if (g[u].empty()) {
          v = -1;
          path.push_back(u);
          s.pop_back();
          break;
        }
        long long v = g[u].back().first;
        long long w = g[u].back().second;
        g[u].pop_back();
        if (visited[w]) {
          continue;
        }
        visited[w] = true;
        s.push_back(v);
        break;
      }
    }
    if (v1 != -1) {
      for (long long i = 0; i < (long long)(path.size() - 1); ++i) {
        if ((path[i] == v1 && path[i + 1] == v2) ||
            (path[i] == v2 && path[i + 1] == v1)) {
          vector<long long> res;
          for (long long j = (long long)(i + 1); j < (long long)(path.size());
               ++j) {
            res.push_back(path[j]);
          }
          for (long long j = (long long)(1); j < (long long)(i + 1); ++j) {
            res.push_back(path[j]);
          }
          path = res;
          break;
        }
      }
    }
    for (long long u = 0; u < (long long)(n); ++u) {
      if (!g[u].empty()) {
        return false;
      }
    }
    return true;
  }
};
long long getPref(long long c, long long i) { return ((1 << i) - 1) & c; }
struct PairHash {
  int operator()(const pair<long long, long long>& pp) const {
    const int p = 31;
    const int m = 1e9 + 9;
    long long l = pp.first;
    long long r = (pp.second * p) % m;
    return (l + r) % m;
  }
};
void solve() {
  long long n;
  cin >> n;
  vector<pair<long long, long long>> colors(n);
  for (long long i = 0; i < (long long)(n); ++i) {
    long long a, b;
    cin >> a >> b;
    colors[i] = {a, b};
  }
  vector<long long> res;
  for (long long i = (long long)(1); i < (long long)(2 * n + 1); ++i) {
    res.push_back(i);
  }
  long long lowest = 0;
  long long l = 1;
  long long r = 20;
  while (l <= r) {
    long long i = l + (r - l) / 2;
    unordered_map<pair<long long, long long>,
                  vector<pair<long long, long long>>, PairHash>
        nodes2Idx;
    Euler e(1 << i);
    for (long long j = 0; j < (long long)(n); ++j) {
      long long pref1 = getPref(colors[j].first, i);
      long long pref2 = getPref(colors[j].second, i);
      long long idx1 = 2 * j + 1;
      long long idx2 = 2 * j + 2;
      if (pref1 > pref2) {
        swap(pref1, pref2);
        swap(idx1, idx2);
      }
      nodes2Idx[{pref1, pref2}].push_back({idx1, idx2});
      e.addEdge(pref1, pref2);
    }
    if (!e.compute()) {
      r = i - 1;
      continue;
    }
    l = i + 1;
    lowest = i;
    assert(e.path.size() == n + 1);
    for (long long j = 0; j < (long long)(n); ++j) {
      long long pref1 = e.path[j];
      long long pref2 = e.path[j + 1];
      bool swaped = false;
      if (pref1 > pref2) {
        swap(pref1, pref2);
        swaped = true;
      }
      auto it = nodes2Idx.find({pref1, pref2});
      pair<long long, long long> idxPair = it->second.back();
      it->second.pop_back();
      long long idx1 = idxPair.first;
      long long idx2 = idxPair.second;
      if (swaped) swap(idx1, idx2);
      res[2 * j] = idx1;
      res[2 * j + 1] = idx2;
    }
  }
  cout << lowest << "\n";
  join(res, " ");
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  solve();
  return 0;
}
