#include <bits/stdc++.h>
using namespace std;
template <class c>
struct rge {
  c b, e;
};
template <class c>
rge<c> range(c i, c j) {
  return rge<c>{i, j};
}
template <class c>
auto dud(c* x) -> decltype(cerr << *x, 0);
template <class c>
char dud(...);
struct debug {
  template <class c>
  debug& operator<<(const c&) {
    return *this;
  }
};
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  vector<int> a(n + 1);
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  vector<pair<int, int>> ans;
  int curr = n;
  queue<pair<int, int>> ones;
  queue<int> twos, threes;
  for (int i = n; i >= 1; i--) {
    if (a[i] == 1) {
      if (curr < 1) {
        cout << -1 << endl;
        return 0;
      }
      ans.push_back({curr, i});
      ones.push({curr, i});
      curr--;
    } else if (a[i] == 2) {
      if (ones.empty()) {
        cout << -1 << endl;
        return 0;
      }
      int r = ones.front().first;
      ones.pop();
      ans.push_back({r, i});
      twos.push(i);
    } else if (a[i] == 3) {
      if (curr < 1) {
        cout << -1 << endl;
        return 0;
      }
      if (!threes.empty()) {
        ans.push_back({curr, i});
        ans.push_back({curr, threes.front()});
        threes.pop();
        threes.push(i);
        curr--;
      } else if (!twos.empty()) {
        ans.push_back({curr, i});
        ans.push_back({curr, twos.front()});
        twos.pop();
        threes.push(i);
        curr--;
      } else if (!ones.empty()) {
        ans.push_back({curr, i});
        ans.push_back({curr, ones.front().second});
        ones.pop();
        threes.push(i);
        curr--;
      } else {
        cout << -1 << endl;
        return 0;
      }
    }
  }
  assert(ans.size() <= 2 * n);
  cout << ans.size() << endl;
  for (auto it : ans) {
    cout << it.first << " " << it.second << endl;
  }
}
