#include <bits/stdc++.h>
using namespace std;
struct rec {
  int x, p;
} a[101];
vector<rec> ans;
int n;
bool was[101];
bool cmp(rec a, rec b) { return a.x > b.x; }
void dfs(int v) {
  was[v] = true;
  for (int i = v + 1; i < n; ++i) {
    if (a[v].x == 0) break;
    if (!was[i]) {
      a[v].x--;
      rec r;
      r.x = a[v].p;
      r.p = a[i].p;
      ans.push_back(r);
      dfs(i);
    }
  }
}
int main() {
  cin >> n;
  for (int i = 0; i < n; ++i) {
    cin >> a[i].x;
    a[i].p = i;
  }
  sort(a + 1, a + n, cmp);
  dfs(0);
  for (int i = 0; i < n; ++i)
    if (!was[i]) {
      cout << -1;
      return 0;
    }
  cout << ans.size() << endl;
  for (int i = 0; i < ans.size(); ++i) {
    cout << ans[i].x + 1 << " " << ans[i].p + 1 << endl;
  }
  return 0;
}
