#include <bits/stdc++.h>
using namespace std;
long long int gcd(long long int a, long long int b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long int lcm(long long int a, long long int b) {
  return a * b / gcd(a, b);
}
long long int fexp(long long int a, long long int b) {
  long long int ans = 1;
  while (b) {
    if (b & 1) ans = ans * a % 1000000007;
    b /= 2;
    a = a * a % 1000000007;
  }
  return ans;
}
long long int inverse(long long int a, long long int p) {
  return fexp(a, p - 2);
}
void ingraph(vector<vector<long long int> >& graph, long long int m) {
  long long int x, y;
  for (long long int i = 0; i < m; i++) {
    cin >> x >> y;
    x--, y--;
    graph[x].push_back(y);
    graph[y].push_back(x);
  }
}
long long int modify(long long int n) {
  long long int res = n;
  res %= 1000000007;
  res += 1000000007;
  res %= 1000000007;
  return res;
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long int t;
  t = 1;
  while (t--) {
    long long int n, k;
    cin >> n >> k;
    vector<long long int> dist[n];
    long long int x;
    for (long long int i = 0; i < n; i++) {
      cin >> x;
      dist[x].push_back(i);
    }
    bool possible = 1;
    long long int source;
    if (dist[0].size() != 1)
      possible = false;
    else
      source = dist[0][0];
    vector<pair<long long int, long long int> > edges;
    if (possible) {
      for (long long int i = 1; i < n; i++) {
        if (dist[i].size() > dist[i - 1].size() * (k - (i != 1))) {
          possible = false;
          break;
        }
        for (long long int j = 0; j < dist[i].size(); j++)
          edges.push_back(
              {dist[i][j] + 1, dist[i - 1][j / (k - (i != 1))] + 1});
      }
    }
    if (!possible)
      cout << -1;
    else {
      cout << edges.size() << endl;
      for (long long int i = 0; i < edges.size(); i++)
        cout << edges[i].first << " " << edges[i].second << endl;
    }
  }
}
