#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const long long INFF = 0x3f3f3f3f3f3f3f3fll;
const long long M = 1e9 + 7;
const long long maxn = 1e6 + 107;
const double pi = acos(-1.0);
const double eps = 0.0000000001;
long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }
template <typename T>
inline void pr2(T x, int k = 64) {
  long long i;
  for (i = 0; i < k; i++) fprintf(stderr, "%d", (x >> i) & 1);
  putchar(' ');
}
template <typename T>
inline void add_(T &A, int B, long long MOD = M) {
  A += B;
  (A >= MOD) && (A -= MOD);
}
template <typename T>
inline void mul_(T &A, long long B, long long MOD = M) {
  A = (A * B) % MOD;
}
template <typename T>
inline void mod_(T &A, long long MOD = M) {
  A %= MOD;
  A += MOD;
  A %= MOD;
}
template <typename T>
inline void max_(T &A, T B) {
  (A < B) && (A = B);
}
template <typename T>
inline void min_(T &A, T B) {
  (A > B) && (A = B);
}
template <typename T>
inline T abs(T a) {
  return a > 0 ? a : -a;
}
inline long long powMM(long long a, long long b, long long mod = M) {
  long long ret = 1;
  for (; b; b >>= 1ll, a = a * a % mod)
    if (b & 1) ret = ret * a % mod;
  return ret;
}
int startTime;
void startTimer() { startTime = clock(); }
void printTimer() {
  fprintf(stderr, "/--- Time: %ld milliseconds ---/\n", clock() - startTime);
}
int A[maxn];
queue<pair<int, int> > q1, q2, q3;
vector<pair<int, int> > ans;
int main() {
  int tot = 0, i, n;
  scanf("%d", &n);
  for (i = 1; i <= n; i++) scanf("%d", &A[i]);
  for (i = n; i >= 1; i--) {
    if (A[i] == 1) {
      ans.push_back(make_pair(++tot, i));
      q1.push(make_pair(tot, i));
    } else if (A[i] == 2) {
      if (!q1.size()) return 0 * puts("-1");
      pair<int, int> v;
      v = q1.front();
      q1.pop();
      ans.push_back(make_pair(v.first, i));
      q2.push(make_pair(v.first, i));
    } else if (A[i] == 3) {
      if (!q1.size() && !q2.size() && !q3.size()) return 0 * puts("-1");
      pair<int, int> v;
      if (q3.size()) {
        v = q3.front();
        q3.pop();
      } else if (q2.size()) {
        v = q2.front();
        q2.pop();
      } else if (q1.size()) {
        v = q1.front();
        q1.pop();
      }
      ans.push_back(make_pair(++tot, v.second));
      ans.push_back(make_pair(tot, i));
      q3.push(make_pair(tot, i));
    }
  }
  printf("%d\n", (int)ans.size());
  for (pair<int, int> v : ans) printf("%d %d\n", n - v.first + 1, v.second);
}
