#include <cmath>
#include <ctime>
#include <iostream>
#include <string>
#include <vector>
#include<cstdio>
#include<sstream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<map>
#include<set>
#include<queue>
#include<cctype>
#include <iomanip>
#include <string>
#include <sstream>

#include <functional>
#include <numeric>
#include <stack>
#include <climits>
#include <float.h>
#include<unordered_map>
#include <bitset>


using namespace std;

#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define rep(i, n) for(int i = 0; i < (n); ++i)
#define f(i,a,b) for(int i=a;i<b;i++)
#define F(i,a,b) for(int i=a;i>=    b;i--)
#define sz(a) ((int)a.size())
#define all(c) c.begin(), c.end()
#define fast ios_base::sync_with_stdio(0);cin.tie(0);
#define dbgs(x) cerr << (#x) << " --> " << (x) << ' '
#define dbg(x) cerr << (#x) << " --> " << (x) << endl
#define sz(a) ((int)a.size())

//#define fast ios_base::sync_with_stdio(0);cin.tie(0) ; 

typedef long long ll;
typedef unsigned long long ull;
typedef pair <ll, ll> pll;
typedef vector<ll> vll;
typedef vector<int> vi;

typedef vector<pair<int, int> > vpii;
typedef map<int, int> mii;
typedef map<ll, ll> mll;
typedef pair<int, int> pii;
typedef pair<string, int> psi;
typedef long double ld;


template <class T> const T& max(const T& a, const T&b, const T&c) { return max(a, max(b, c)); }

template <class T> const T& min(const T& a, const T&b, const T&c) { return min(a, min(b, c)); }

void sc(int &a) { scanf("%d", &a); }void sc(ll &a) { scanf("%lld", &a); }void sc(int& a, int& b) { sc(a); sc(b); }
void sc(ll &a, ll &b) { sc(a); sc(b); }void sc(int& a, int& b, int& c) { sc(a, b); sc(c); }void sc(ll& a, ll& b, ll& c) { sc(a, b); sc(c); }
void prl(int a) { printf("%d\n", a); }void prl(ll a) { printf("%lld\n", a); }void prl() { printf("\n"); }
void prs(int a) { printf("%d ", a); }void prs(ll a) { printf("%lld ", a); }


int gcd(int a, int b) { if (b == 0) return a; return gcd(b, a % b); }

ll lcm(ll a, ll b) { return (a * b) / gcd(a, b); }

ll poww(ll a, ll b) {

	if (b == 0) return 1;

	ll tmp = poww(a, b / 2);

	return (b & 1 ? a * tmp * tmp : tmp * tmp);

}



ll sumOfDigs(string s) { ll sum = 0; for (int i = 0; i < s.length(); i++) sum += s[i] - '0'; return sum; }

ll sumOfDigs(ll n) { return (n < 10 ? n : n % 10 + sumOfDigs(n / 10)); }

string itos(ll i) { string s = ""; 	while (i) { s += char(i % 10 + '0'); i /= 10; } reverse(all(s));  return s; }

ll stoi(string &s) { ll tot = 0; for (int i = (int)s.length() - 1, j = 1; i >= 0; i--, j *= 10) { tot += j * (s[i] + '0'); }  return tot; }


int months[13] = { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };


using namespace std;

#define PI acos((ld)-1.0) 





// printf("%.8lf\n", sum * 1.0 / n);  // precision setting


ll mod = 1e9 + 7;

void tt() {

#ifndef online	_judge
	freopen("test.txt", "r", stdin);
	//freopen("output.txt", "w", stdout);

#endif
}

ll modpower(ll x, ll y, ll p)
{

	x %= mod;

	if (!y) return 1;

	ll res = 1;

	if (y & 1) { res *= x;  res %= p; }

	ll z = modpower(x, y / 2, p);

	z %= p;

	z *= z;

	z %= mod;

	res *= z;

	res %= p;

	return res;

}

pii a[1000 * 1000 + 10];


vector<int> mpp[1000 * 1000 + 10]; 

vector<pii> ans;

int n, k;

int len, start;

void dfs(int curr) {

	if (len >= n) { return;  }

	if ((int)mpp[len].size() == 0 && (int)mpp[len + 1].size() != 0) {   cout << -1 << endl; exit(0); }

	//cout << "HI" << endl; 

	for (int i = 1; i <= k - 1 + (curr == start); i++) {
		if ((int)mpp[len].size()) {

			ans.pb({ curr, mpp[len].back() });
			len++;
			dfs(mpp[len - 1].back());
			len--;
			mpp[len].pop_back();

		}
	}
}

int main() {

	
	//stt();
	fast;
	cin >> n >> k;

	start = -1;

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

		int x;
		cin >> x;
		mpp[x].pb(i);
		if (x == 0) start = i;

	}


	if ((int)mpp[0].size() != 1) { return cout << -1 << endl, 0; }

	
	len++;

	dfs(start);

	mpp[0].clear(); 
	
	
	for (auto t : mpp) if ((int)t.size()) { return cout << -1 << endl, 0; }
	cout << (int)ans.size() << endl;
	for (auto t : ans)cout << t.ff << " " << t.ss << endl;
 
	return 0;

}