#include <bits/stdc++.h>
using namespace std;
mt19937 rnd(time(0));
const long long inf = 0x3f3f3f3f3f3f3f3fLL;
const long long N = 3e5 + 5;
const long long MOD = 1e9 + 7;
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  long long n;
  cin >> n;
  vector<long long> a(n + 1);
  for (long long i = 1; i <= n; i++) cin >> a[i];
  long long r = n + 1, lst = n + 1;
  vector<long long> row(n + 1), o;
  vector<pair<long long, long long>> res;
  for (long long i = n; i > 0; i--) {
    if (a[i] == 0) continue;
    if (a[i] > 1 && r == n + 1) {
      cout << -1;
      return 0;
    }
    if (a[i] == 1) {
      r--;
      res.emplace_back(r, i);
      row[i] = r;
      o.push_back(i);
    } else if (a[i] == 2) {
      if (o.empty()) {
        cout << -1;
        return 0;
      }
      long long x = o.back();
      o.pop_back();
      row[i] = row[x];
      res.emplace_back(row[i], i);
      lst = i;
    } else if (a[i] == 3) {
      r--;
      long long x = n + 1;
      if (lst != n + 1)
        x = lst;
      else if (!o.empty()) {
        x = o.back();
        o.pop_back();
      } else {
        cout << -1;
        return 0;
      }
      res.emplace_back(r, x);
      res.emplace_back(r, i);
      row[i] = r;
      lst = i;
    }
  }
  cout << res.size() << '\n';
  for (auto i : res) cout << i.first << ' ' << i.second << '\n';
  return 0;
}
