#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize(3)
namespace IO {
char buf[1 << 21], *p1 = buf, *p2 = buf, obuf[1 << 23], *O = obuf;
inline int read() {
  int x = 0, sign = 0;
  char s = getchar();
  while (!isdigit(s)) sign |= s == '-', s = getchar();
  while (isdigit(s)) x = (x << 1) + (x << 3) + (s - '0'), s = getchar();
  return sign ? -x : x;
}
void print(int x) {
  if (x > 9) print(x / 10);
  (*O++ = x % 10 + '0');
}
}  // namespace IO
namespace math {
const int mod = 1e9 + 7;
const int maxn = 1e6 + 5;
long long ksm(long long a, long long b) {
  long long s = 1, m = a;
  while (b) {
    if (b & 1) s = s * m % mod;
    m = m * m % mod, b >>= 1;
  }
  return s;
}
long long inv(long long x) { return ksm(x, mod - 2); }
long long fc[maxn], ifc[maxn];
void init_m(int n) {
  fc[0] = 1;
  for (int i = 1; i <= n; i++) fc[i] = fc[i - 1] * i % mod;
  ifc[n] = inv(fc[n]);
  for (int i = n - 1; i >= 0; i--) ifc[i] = ifc[i + 1] * (i + 1) % mod;
}
long long C(long long n, long long m) {
  return fc[n] * ifc[m] % mod * ifc[n - m] % mod;
}
}  // namespace math
using namespace IO;
const int N = (1 << 20) + 5;
struct edge {
  int u, v, to, d;
} vv[N];
int c, hd[N], nxt[N];
void add(int u, edge t) { nxt[++c] = hd[u], hd[u] = c, vv[c] = t; }
int n, a[N], b[N], v[N];
vector<int> ans;
void cir(int id) {
  for (int i = hd[id]; i; i = hd[id]) {
    hd[id] = nxt[i];
    edge it = vv[i];
    if (!v[it.d])
      v[it.d] = 1, cir(it.to), ans.emplace_back(it.v), ans.emplace_back(it.u);
  }
}
void end(int k) {
  int bs = (1 << k) - 1;
  memset(v, 0, sizeof(v));
  for (int i = 1; i <= n; i++) {
    int c = a[i] & bs, d = b[i] & bs;
    add(c, (edge){i * 2 - 1, i * 2, d, i}),
        add(d, (edge){i * 2, i * 2 - 1, c, i});
  }
  cir(a[1] & bs);
  cout << k << endl;
  for (int it : ans) cout << it << " ";
  exit(0);
}
vector<int> e[N];
void dfs(int id) {
  v[id] = 1;
  for (int it : e[id])
    if (!v[it]) dfs(it);
}
int block(int k) {
  int c = 0;
  for (int i = 0; i < 1 << k; i++)
    if (e[i].size() && !v[i]) {
      c++, dfs(i);
    }
  return c;
}
int main() {
  n = read();
  for (int i = 1; i <= n; i++) a[i] = read(), b[i] = read();
  for (int i = 1, bs = 1; i <= 20;
       i++, bs = (bs << 1) + 1, memset(v, 0, sizeof(v))) {
    for (int j = 0; j < 1 << i; j++) e[j].clear();
    for (int j = 1; j <= n; j++) {
      int c = a[j] & bs, d = b[j] & bs;
      e[c].emplace_back(d), e[d].emplace_back(c);
    }
    int od = 0;
    for (int j = 0; j < (1 << i); j++) od |= e[j].size() & 1;
    if (od || block(i) != 1) end(i - 1);
  }
  end(20);
  return 0;
}
