#include <bits/stdc++.h>
using namespace std;
const long long N = 2e5;
const long long M = 25;
const long long block = 8000;
const long long big = 1e18;
const long long mod = 998244353;
mt19937_64 srnd(chrono::steady_clock::now().time_since_epoch().count());
long long n, k;
long long a[N];
long long zanim[N];
int main() {
  ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  set<long long> z[2];
  vector<pair<long long, long long> > ans;
  long long curL = 1;
  for (int i = n - 1; i >= 0; i--) {
    if (a[i] == 0) continue;
    if (a[i] == 1)
      z[0].insert(curL++), ans.push_back(make_pair(curL - 1, i + 1)),
          zanim[curL - 1] = i + 1;
    if (a[i] == 2) {
      if (!z[0].size()) cout << -1, exit(0);
      pair<long long, long long> h = {*z[0].begin(), i + 1};
      z[1].insert(i + 1);
      z[0].erase(h.first);
      ans.push_back(h);
    }
    if (a[i] == 3) {
      if (!z[1].size() && !z[0].size()) cout << -1, exit(0);
      if (z[1].size()) {
        long long h = *z[1].begin();
        ans.push_back(make_pair(curL, i + 1));
        ans.push_back(make_pair(curL, h));
        z[1].erase(h);
        z[1].insert(i + 1);
        curL++;
      } else {
        long long h = *z[0].begin();
        z[0].erase(z[0].begin());
        ans.push_back(make_pair(curL, i + 1));
        ans.push_back(make_pair(curL, zanim[h]));
        z[1].insert(i + 1);
        curL++;
      }
    }
  }
  cout << ans.size() << '\n';
  for (auto u : ans) cout << n - u.first + 1 << " " << u.second << '\n';
}
