#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  int n, k;
  cin >> n >> k;
  vector<int> *dist = new vector<int>[n];
  int len = 0;
  for (int i = 0; i < n; i++) {
    int d;
    cin >> d;
    dist[d].push_back(i);
    len = max(len, d);
  }
  if (dist[0].size() != 1) {
    cout << -1;
    return 0;
  }
  for (int i = 1; i <= len; i++) {
    if (dist[i].empty() ||
        dist[i].size() > (long long)dist[i - 1].size() * (k - (i != 1))) {
      cout << -1;
      return 0;
    }
  }
  cout << n - 1 << endl;
  for (int i = 1; i <= len; i++) {
    int counter = 0;
    int index = 0;
    for (int child : dist[i]) {
      if (counter == k - (i != 1)) {
        counter = 0;
        index++;
      }
      cout << dist[i - 1][index] + 1 << " " << child + 1 << endl;
      counter++;
    }
  }
  return 0;
}
