#include <bits/stdc++.h>
#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
using namespace std;
template <class S, class T>
pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) {
  return pair<S, T>(s.first + t.first, s.second + t.second);
}
template <class S, class T>
pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) {
  return pair<S, T>(s.first - t.first, s.second - t.second);
}
template <class S, class T>
ostream &operator<<(ostream &os, pair<S, T> p) {
  os << "(" << p.first << ", " << p.second << ")";
  return os;
}
template <class T>
bool chmax(T &a, const T &b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T &a, const T &b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
const long long inf = 1000000001;
const long long INF = (long long)1e18 + 1;
const long double pi = 3.1415926535897932384626433832795028841971L;
vector<int> dx = {1, 0, -1, 0}, dy = {0, 1, 0, -1};
const long long MOD = 1000000007;
void solve() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < (int)(n); i++) cin >> a[i];
  queue<pair<int, int> > qu1, qu2;
  int nex = n - 1;
  vector<pair<int, int> > ans;
  for (int i = (int)(n)-1; i >= 0; i--) {
    if (a[i] == 0)
      continue;
    else if (a[i] == 1) {
      if (nex == -1) {
        cout << -1 << endl;
        return;
      }
      ans.push_back(pair<int, int>(nex, i));
      qu1.push(pair<int, int>(nex, i));
      nex--;
    } else if (a[i] == 2) {
      if (qu1.size() == 0) {
        cout << -1 << endl;
        return;
      }
      pair<int, int> temp = qu1.front();
      qu1.pop();
      int y = temp.first;
      ans.push_back(pair<int, int>(y, i));
      qu2.push(pair<int, int>(y, i));
    } else {
      if (qu2.size() == 0 and qu1.size() == 0) {
        cout << -1 << endl;
        return;
      }
      pair<int, int> temp;
      if (qu2.size() > 0) {
        temp = qu2.front();
        qu2.pop();
      } else {
        temp = qu1.front();
        qu1.pop();
      }
      int y = temp.first, x = temp.second;
      ans.push_back(pair<int, int>(nex, i));
      ans.push_back(pair<int, int>(nex, x));
      qu2.push(pair<int, int>(nex, i));
      nex--;
    }
  }
  cout << ans.size() << endl;
  for (int i = 0; i < (int)(ans.size()); i++) {
    cout << ans[i].first + 1 << " " << ans[i].second + 1 << '\n';
  }
}
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  ;
  int t = 1;
  while (t--) solve();
}
