#pragma comment(linker, "/STACK:1024000000,1024000000")

#include <cstdio>

#include <string>

#include <cstdlib>

#include <cmath>

#include <iostream>

#include <cstring>

#include <set>

#include <queue>

#include <algorithm>

#include <vector>

#include <map>

#include <cctype>

#include <cmath>

#include <stack>

#include <sstream>

#define debug() puts("++++");

#define gcd(a, b) __gcd(a, b)

#define lson l,m,rt<<1

#define rson m+1,r,rt<<1|1

#define freopenr freopen("in.txt", "r", stdin)

#define freopenw freopen("out.txt", "w", stdout)

using namespace std;

typedef long long LL;

typedef unsigned long long ULL;

typedef pair<int, int> P;

const int INF = 0x3f3f3f3f;

const LL LNF = 1e16;

const double inf = 0x3f3f3f3f3f3f;

const double PI = acos(-1.0);

const double eps = 1e-8;

const int maxn = 100000 + 10;

const int mod = 100000000;

const int dr[] = {-1, 0, 1, 0};

const int dc[] = {0, 1, 0, -1};

const char *de[] = {"0000", "0001", "0010", "0011", "0100", "0101", "0110", "0111", "1000", "1001", "1010", "1011", "1100", "1101", "1110", "1111"};

int n, m;

const int mon[] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

const int monn[] = {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

inline bool is_in(int r, int c){

  return r >= 0 && r < n && c >= 0 && c < m;

}

vector<int> G[maxn];

int main(){

  scanf("%d %d", &n, &m);

  int rt = -1, cnt = 0;

  int mmax = 0;

  for(int i = 1; i <= n; ++i){

    int x;

    scanf("%d", &x);

    G[x].push_back(i);

    if(x == 0)  rt = i;

    mmax = max(mmax, x);

  }

  if(G[1].size() > G[0].size() * m || G[0].size() != 1){

    printf("-1\n");

    return 0;

  }

  for(int i = 2; i <= mmax; ++i)

    if(G[i].size() > (LL)G[i-1].size() * (m-1)){

      printf("-1\n");

      return 0;

    }

  printf("%d\n", n-1);

  for(int i = 0; i < mmax; ++i){

    bool ok = true;

    int p = 0;

    for(int j = 0; j < G[i].size() && ok; ++j){

      int u = G[i][j];

      for(int k = (i != 0); k < m && ok; ++k){

        printf("%d %d\n", u, G[i+1][p++]);

        if(p == G[i+1].size())  ok = false;

      }

    }

  }

  return 0;

}