#include <bits/stdc++.h>
using namespace std;
void err(istream_iterator<string> it) { cout << endl; }
template <typename T, typename... Args>
void err(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " = " << a << " ";
  err(++it, args...);
}
template <typename T>
inline void read(T& x) {
  x = 0;
  T f = 1;
  char ch = getchar();
  while (!isdigit(ch)) f = (ch == '-') ? -1 : 1, ch = getchar();
  while (isdigit(ch)) x = x * 10 + ch - 48, ch = getchar();
  x *= f;
}
template <typename T, typename... Args>
inline void read(T& x, Args&... args) {
  read(x), read(args...);
}
constexpr int MAXN = 1e5 + 5, MOD = 1e9 + 7;
int n, cnt;
array<int, MAXN> a;
array<vector<pair<int, int>>, MAXN> p;
vector<int> id[4];
int main() {
  ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  ;
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  int high = n + 1;
  for (int i = n; i >= 1; i--) {
    if (a[i] == 0)
      continue;
    else if (a[i] == 1) {
      if (high < 1) return cout << -1 << "\n", 0;
      p[i].emplace_back(i, --high), id[1].push_back(i), cnt++;
    } else if (a[i] == 2) {
      if (id[1].size() == 0) return cout << -1 << "\n", 0;
      int tmp = id[1].back();
      id[1].pop_back();
      p[i].emplace_back(i, p[tmp][0].second);
      id[2].push_back(i), cnt++;
    } else {
      if (id[3].size()) {
        int tmp = id[3].back();
        id[3].pop_back();
        if (high < 1) return cout << -1 << "\n", 0;
        p[i].emplace_back(i, --high);
        p[i].emplace_back(p[tmp][0].first, high);
      } else if (id[2].size()) {
        int tmp = id[2].back();
        id[2].pop_back();
        if (high < 1) return cout << -1 << "\n", 0;
        p[i].emplace_back(i, --high);
        p[i].emplace_back(p[tmp][0].first, high);
      } else if (id[1].size()) {
        int tmp = id[1].back();
        id[1].pop_back();
        if (high < 1) return cout << -1 << "\n", 0;
        p[i].emplace_back(i, --high);
        p[i].emplace_back(p[tmp][0].first, high);
      } else
        return cout << -1 << "\n", 0;
      id[3].push_back(i);
      cnt += 2;
    }
  }
  cout << cnt << "\n";
  for (int i = n; i >= 1; i--)
    for (auto& j : p[i]) cout << j.second << " " << j.first << "\n";
  return 0;
}
