#include <bits/stdc++.h>
using namespace std;
vector<pair<long long int, long long int> > dis(100005), edge;
long long int cnt[100005];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long int t = 0, n, e, x, y, z, i, j, k, g, q, ans = 0, sum = 0, c = 0, h;
  cin >> n >> t;
  for (i = 1; i <= n; i++) {
    cin >> x;
    dis[i].first = x;
    dis[i].second = i;
  }
  sort(dis.begin() + 1, dis.begin() + n + 1);
  if (dis[1].first != 0) {
    cout << "-1\n";
    return 0;
  }
  if (n == 1) {
    cout << "0\n";
    return 0;
  }
  if (dis[1].first == dis[2].first) {
    cout << "-1\n";
    return 0;
  }
  vector<long long int> v1, v2;
  v1.push_back(dis[1].second);
  i = 2;
  long long int next = 1;
  while (i <= n) {
    for (j = i; j <= n; j++) {
      if (dis[j].first != next) break;
      v2.push_back(dis[j].second);
    }
    i = j;
    long long int s1 = v1.size();
    long long int s2 = v2.size();
    for (j = 0, k = 0; j < s2 && k < s1;) {
      if (cnt[v1[k]] == t) {
        k++;
        continue;
      }
      edge.push_back(make_pair(v1[k], v2[j]));
      cnt[v1[k]]++;
      cnt[v2[j]]++;
      if (cnt[v1[k]] == t) k++;
      j++;
    }
    if (j != s2) {
      cout << "-1\n";
      return 0;
    }
    v1.clear();
    next++;
    for (j = 0; j < s2; j++) {
      v1.push_back(v2[j]);
    }
    v2.clear();
  }
  ans = edge.size();
  cout << ans << "\n";
  for (i = 0; i < edge.size(); i++) {
    cout << edge[i].first << " " << edge[i].second << "\n";
  }
  return 0;
}
