#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long int n;
  cin >> n;
  vector<long long int> a(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  long long int cr = n;
  bool p = 1;
  vector<pair<long long int, long long int>> ans;
  stack<pair<long long int, long long int>> ones;
  stack<pair<long long int, long long int>> twos;
  stack<pair<long long int, long long int>> threes;
  for (int i = n - 1; i >= 0; i--) {
    if (a[i] == 1) {
      cr--;
      if (cr < 0) {
        p = 0;
        break;
      }
      ans.push_back({cr, i});
      ones.push({cr, i});
    }
    if (a[i] == 2) {
      if (ones.empty()) {
        p = 0;
        break;
      }
      ans.push_back({ones.top().first, i});
      twos.push({ones.top().first, i});
      ones.pop();
    }
    if (a[i] == 3) {
      cr--;
      if (cr < 0 || (threes.empty() && twos.empty() && ones.empty())) {
        p = 0;
        break;
      }
      if (!threes.empty()) {
        ans.push_back({cr, i});
        ans.push_back({cr, threes.top().second});
        threes.pop();
      } else if (!twos.empty()) {
        ans.push_back({cr, i});
        ans.push_back({cr, twos.top().second});
        twos.pop();
      } else {
        ans.push_back({cr, i});
        ans.push_back({cr, ones.top().second});
        ones.pop();
      }
      threes.push({cr, i});
    }
  }
  if (!p) {
    cout << -1 << endl;
    return 0;
  }
  cout << ans.size() << endl;
  for (auto x : ans) {
    cout << x.first + 1 << " " << x.second + 1 << endl;
  }
}
