#include <bits/stdc++.h>
using namespace std;
template <typename T, typename U>
inline void smin(T &a, U b) {
  if (a > b) a = b;
}
template <typename T, typename U>
inline void smax(T &a, U b) {
  if (a < b) a = b;
}
template <typename T>
inline void gn(T &first) {
  char c, sg = 0;
  while (c = getchar(), (c > '9' || c < '0') && c != '-')
    ;
  for ((c == '-' ? sg = 1, c = getchar() : 0), first = 0; c >= '0' && c <= '9';
       c = getchar())
    first = (first << 1) + (first << 3) + c - '0';
  if (sg) first = -first;
}
template <class T, class T1>
inline void gn(T &first, T1 &second) {
  gn(first);
  gn(second);
}
template <class T, class T1, class T2>
inline void gn(T &first, T1 &second, T2 &z) {
  gn(first);
  gn(second);
  gn(z);
}
template <typename T>
inline void print(T first) {
  if (first < 0) {
    putchar('-');
    return print(-first);
  }
  if (first < 10) {
    putchar('0' + first);
    return;
  }
  print(first / 10);
  putchar(first % 10 + '0');
}
template <typename T>
inline void println(T first) {
  print(first), putchar('\n');
}
template <typename T>
inline void printsp(T first) {
  print(first), putchar(' ');
}
template <class T, class T1>
inline void print(T first, T1 second) {
  printsp(first), println(second);
}
template <class T, class T1, class T2>
inline void print(T first, T1 second, T2 z) {
  printsp(first), printsp(second), println(z);
}
int power(int a, int b, int m, int ans = 1) {
  for (; b; b >>= 1, a = 1LL * a * a % m)
    if (b & 1) ans = 1LL * ans * a % m;
  return ans;
}
pair<pair<int, int>, int> p[(1 << 16)];
set<pair<pair<int, int>, int> > s;
vector<pair<int, int> > edge;
int main() {
  int n;
  gn(n);
  for (int i = 0; i < n; i++) {
    gn(p[i].first.first, p[i].first.second);
    p[i].second = i;
    s.insert(p[i]);
  }
  for (int i = 0; i < n; i++) {
    pair<pair<int, int>, int> q = *s.begin();
    s.erase(s.begin());
    if (q.first.first == 0) continue;
    if (q.first.first == 1) {
      int u = q.second;
      int v = q.first.second;
      edge.push_back(pair<int, int>(u, v));
      s.erase(p[v]);
      p[v].first.first--;
      p[v].first.second ^= u;
      s.insert(p[v]);
    }
  }
  println(edge.size());
  for (int i = 0; i < edge.size(); i++) print(edge[i].first, edge[i].second);
  return 0;
}
