#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<ll,ll> P;
#define REP(i,n) for(ll i=0;i<ll(n);i++)







vector<ll> a[1010];





int main(void){
	cin.tie(nullptr);  ios_base::sync_with_stdio(false);
	ll i,j,k;




	ll N,K;
	cin >> N >> K;

	vector<ll> p(N+1,0);
	for(i=1;i<=N;i++) cin >> p[i];

	vector<ll> c(N+1,0);


	for(i=1;i<=N-K;i++){
		if(c[i]==1) break;
		for(j=i;j<=N;j+=K){
			if((p[j]-i)%K!=0){
				cout << -1 << endl;
				return 0;

			}

			a[i].push_back(j);
			c[j]=1;
		}
	}




	ll ans=0;
	vector<P> d;
	for(i=1;i<=N-K;i++){
		vector<ll> v=a[i];
		ll n=v.size();
		    for(k=0;k<n;k++){
		        for(j=0;j<n-1;j++){

		            if(p[v[j]]>p[v[j+1]]){

		                d.push_back(P(v[j],v[j+1]));
		                swap(p[v[j]],p[v[j+1]]);

		                ans++;
		            }
		        }
		    }


	}


	cout << ans << endl;
	REP(i,d.size()){
		cout << d[i].first << ' ' << d[i].second << endl;
	}









	return 0;

}