#include <bits/stdc++.h>
using namespace std;
inline void read(int &x) {
  int v = 0, f = 1;
  char c = getchar();
  while (!isdigit(c) && c != '-') c = getchar();
  if (c == '-')
    f = -1;
  else
    v = (c & 15);
  while (isdigit(c = getchar())) v = (v << 1) + (v << 3) + (c & 15);
  x = v * f;
}
inline void read(long long &x) {
  long long v = 0ll, f = 1ll;
  char c = getchar();
  while (!isdigit(c) && c != '-') c = getchar();
  if (c == '-')
    f = -1;
  else
    v = (c & 15);
  while (isdigit(c = getchar())) v = (v << 1) + (v << 3) + (c & 15);
  x = v * f;
}
inline void readc(char &x) {
  char c;
  while (((c = getchar()) == ' ') || c == '\n')
    ;
  x = c;
}
int n, m, i, j, a[100005], cnt;
vector<pair<int, int> > ans;
vector<pair<int, int> > p1, p2;
int main() {
  read(n);
  for (((i)) = (1); ((i)) <= ((n)); ((i))++) read(a[i]);
  for (((i)) = ((n)); ((i)) >= (1); ((i))--) {
    if (a[i] == 1) {
      p1.push_back(make_pair(i, ++cnt));
      ans.push_back(make_pair(i, cnt));
    }
    if (a[i] == 2) {
      if (p1.empty()) {
        puts("-1");
        return 0;
      }
      p2.push_back(make_pair(i, p1.back().second));
      ans.push_back(make_pair(i, p1.back().second));
      p1.pop_back();
    }
    if (a[i] == 3) {
      if (!p2.empty()) {
        ans.push_back(make_pair(p2.back().first, ++cnt));
        ans.push_back(make_pair(i, cnt));
        p2.pop_back();
        p2.push_back(ans.back());
        continue;
      }
      if (!p1.empty()) {
        ans.push_back(make_pair(p1.back().first, ++cnt));
        ans.push_back(make_pair(i, cnt));
        p1.pop_back();
        p2.push_back(ans.back());
        continue;
      }
      puts("-1");
      return 0;
    }
  }
  printf("%d\n", ans.size());
  for (__typeof((ans).begin()) it = (ans).begin(); it != (ans).end(); it++)
    printf("%d %d\n", n + 1 - it->second, it->first);
  return 0;
}
