#include <bits/stdc++.h>
const long double PI(acosl(-1.0));
long double eps = 1e-9;
using namespace std;
using namespace std;
const int N = 1000000;
int main() {
  std::ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int n;
  cin >> n;
  vector<pair<int, int> > p(n);
  int s = 0;
  for (int i(0); i < n; i++) {
    int x;
    cin >> x;
    s += x;
    p[i] = {x, i + 1};
  }
  sort(++p.begin(), p.end());
  reverse(++p.begin(), p.end());
  vector<pair<int, int> > ans;
  int j = 1;
  int i = 0;
  while (j < n) {
    while (j < n && p[i].first > 0) {
      ans.push_back({p[i].second, p[j].second});
      p[i].first--;
      j++;
    }
    i++;
    if (i == j) break;
  }
  if (j < n) {
    cout << -1;
    return 0;
  }
  cout << ans.size() << endl;
  for (int i(0); i < ans.size(); i++)
    cout << ans[i].first << " " << ans[i].second << "\n";
}
