#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
vector<long long> primeFact(long long n) {
  vector<long long> ans;
  for (long long t = 2; t <= sqrt(n); t++) {
    if (n % t == 0) {
      ans.push_back(t);
      while (n % t == 0) {
        n = n / t;
      }
    }
  }
  if (n > 2) ans.push_back(n);
  return ans;
}
vector<long long> fact(long long n) {
  vector<long long> ans;
  for (long long i = 1; i <= sqrt(n); i++) {
    if (n % i == 0) {
      if (i * i == n) {
        ans.push_back(i);
      } else {
        ans.push_back(i);
        ans.push_back(n / i);
      }
    }
  }
  return ans;
}
long long powerr(long long base, long long exp, long long mod) {
  if (exp == 0) return 1;
  long long t = powerr(base, exp / 2, mod) % mod;
  if (exp % 2 == 0) {
    return (t % mod * t % mod) % mod;
  } else {
    return (t % mod * t % mod * base % mod) % mod;
  }
}
void fastio() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}
vector<pair<long long, long long> > ans;
void dfs(vector<pair<long long, long long> > &a, long long node,
         vector<bool> &vis, long long n) {
  vis[node] = true;
  long long t = 0;
  for (long long i = 0; i < a[node].first; i++) {
    t = node + 1;
    while (vis[t] == true && t < n) {
      t++;
    }
    if (t < n) {
      ans.push_back({a[node].second, a[t].second});
      dfs(a, t, vis, n);
    } else {
      break;
    }
  }
  a[node].first = 0;
}
int32_t main() {
  fastio();
  long long n;
  cin >> n;
  vector<pair<long long, long long> > a;
  long long p;
  for (long long i = 0; i < n; i++) {
    cin >> p;
    a.push_back({p, i + 1});
  }
  sort(a.begin() + 1, a.end());
  reverse(a.begin() + 1, a.end());
  vector<bool> vis(n + 10, false);
  dfs(a, 0, vis, n);
  for (long long i = 0; i < n; i++) {
    if (vis[i] == false) {
      cout << -1 << "\n";
      return 0;
    }
  }
  cout << ans.size() << "\n";
  for (long long i = 0; i < ans.size(); i++) {
    cout << ans[i].first << " " << ans[i].second << "\n";
  }
  return 0;
}
