#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const ll mod = 1e9 + 7;
ll fac[1000001];
void pre() {
  fac[0] = fac[1] = 1;
  for (int i = 2; i <= 1000001; i++) {
    fac[i] = (fac[i - 1] * 1LL * i) % mod;
  }
}
ll binpower(ll a, ll n) {
  ll res = 1;
  while (n) {
    if (n % 2) res = (res * 1LL * a) % mod;
    n /= 2;
    a = (a * 1LL * a) % mod;
  }
  return res;
}
ll nCrmod(ll n, ll r) {
  ll res = fac[n];
  res = (res * 1LL * binpower(fac[r], mod - 2)) % mod;
  res = (res * 1LL * binpower(fac[n - r], mod - 2)) % mod;
  return res;
}
long long ncr(int n, int r) {
  if (r > n - r) r = n - r;
  long long ans = 1;
  int i;
  for (i = 1; i <= r; i++) {
    ans *= n - r + i;
    ans /= i;
  }
  return ans;
}
ll modexp(ll a, ll b, ll m) {
  if (b == 0) return 1;
  if (b % 2 == 0) {
    ll y = modexp(a, b / 2, m);
    return (y * y) % m;
  } else {
    return ((a % m) * modexp(a, b - 1, m)) % m;
  }
}
ll modinv(ll a, ll m) { return modexp(a, m - 2, m); }
void SieveOfEratosthenes(ll n) {
  bool prime[n + 1];
  memset(prime, true, sizeof(prime));
  for (ll p = 2; p * p <= n; p++) {
    if (prime[p] == true) {
      for (ll i = p * p; i <= n; i += p) prime[i] = false;
    }
  }
  for (ll p = 2; p <= n; p++)
    if (prime[p]) cout << p << " ";
}
vector<int> Centroid(vector<int> g[], int n) {
  vector<int> centroid;
  vector<int> sz(n + 1);
  function<void(int, int)> dfs = [&](int u, int prev) {
    sz[u] = 1;
    bool is_centroid = true;
    for (auto v : g[u])
      if (v != prev) {
        dfs(v, u);
        sz[u] += sz[v];
        if (sz[v] > n / 2) is_centroid = false;
      }
    if (n - sz[u] > n / 2) is_centroid = false;
    if (is_centroid) centroid.push_back(u);
  };
  dfs(1, 0);
  return centroid;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int N, M;
  cin >> N;
  int D[N], S[N];
  queue<int> q;
  M = 0;
  for (int i = 0; i < N; i++) {
    cin >> D[i] >> S[i];
    M += D[i];
    if (D[i] == 1) q.push(i);
  }
  M /= 2;
  cout << M << '\n';
  while (!q.empty()) {
    int i = q.front();
    q.pop();
    if (D[i] == 0) continue;
    assert(D[i] == 1);
    int j = S[i];
    cout << i << ' ' << j << '\n';
    D[i]--;
    D[j]--;
    S[j] ^= i;
    S[i] ^= j;
    if (D[j] == 1) q.push(j);
  }
  return 0;
}
