#include <bits/stdc++.h>
using namespace std;
const int MOD = 998244353;
const int N = 200005;
const double PI = 4 * atan(1);
int dx[] = {1, -1, 0, 0, 1, -1, 1, -1};
int dy[] = {0, 0, 1, -1, 1, 1, -1, -1};
long long gcd(long long a, long long b) { return (b == 0 ? a : gcd(b, a % b)); }
long long lcm(long long a, long long b) { return a * (b / gcd(a, b)); }
bool issquare(long long w) { return trunc(sqrt(w)) * trunc(sqrt(w)) == w; }
bool isprime(long long u) {
  for (long long i = 2; i <= (int)sqrt(u); i++) {
    if (u % i == 0) return 0;
  }
  return 1;
}
long long mod(long long to_mod) {
  to_mod %= MOD;
  while (to_mod < 0) to_mod += MOD;
  return to_mod % MOD;
}
long long moduloMultiplication(long long a, long long b, long long mod) {
  long long res = 0;
  a %= mod;
  while (b) {
    if (b & 1) res = (res + a) % mod;
    a = (2 * a) % mod;
    b >>= 1;
  }
  return res;
}
long long power(long long x, long long y) {
  long long res = 1;
  x = x;
  while (y > 0) {
    if (y & 1) res = (res * x);
    y = y >> 1;
    x = (x * x);
  }
  return res;
}
long long n, t, l, r, ans, u, v, x, y;
long long a[N];
map<long long, vector<long long> > adj;
vector<pair<long long, long long> > vv;
set<pair<long long, long long> > vvv;
priority_queue<pair<pair<long long, long long>, long long> > pq;
bool vis[70000];
int main() {
  memset(vis, 0, sizeof(vis));
  scanf("%lld", &n);
  for (int i = 0; i < n; i++) {
    scanf("%lld%lld", &x, &y);
    if (x == 1) pq.push(make_pair(make_pair(-x, y), i));
    vv.push_back(make_pair(x, y));
  }
  ans = 0;
  while (!pq.empty()) {
    pair<pair<long long, long long>, long long> p = pq.top();
    pq.pop();
    vis[p.second] = 1;
    u = p.second;
    v = p.first.second;
    long long nb = p.first.first;
    for (auto t : adj[u]) {
      v = v ^ t;
      nb++;
    }
    if (nb != -1) continue;
    adj[v].push_back(u);
    adj[u].push_back(v);
    vvv.insert((make_pair(min(u, v), max(u, v))));
    ans++;
    pq.push(make_pair(make_pair(-vv[v].first, vv[v].second), v));
  }
  printf("%lld\n", ans);
  for (auto t : vvv) {
    printf("%lld %lld\n", t.first, t.second);
  }
  return 0;
}
