// LUOGU_RID: 98918259
#include<bits/stdc++.h>
#define pii pair<int,int>
using namespace std;
const int MAXN=101;
pii ans[MAXN];
vector <int> p[MAXN];
signed main() {
	int n,m;
	scanf("%d%d",&n,&m);
	vector <pii> vec;
	for(int i=1;i<=n;++i) vec.push_back(make_pair(0,i));
	int t=ceil((double)n/(m-1));
	for(int i=1;i<=n*m;++i) {
		int col;
		scanf("%d",&col);
		p[col].push_back(i); 
	}
	for(int i=1;i<m;++i) {
		for(auto &u:vec) u.first=p[u.second][i];
		sort(vec.begin(),vec.end());
		for(int j=1;!vec.empty()&&j<=t;++j) {
			int k=vec.begin()->second;
			vec.erase(vec.begin());
			ans[k]=make_pair(p[k][i-1],p[k][i]);
		}
	}
	for(int i=1;i<=n;++i) printf("%d %d\n",ans[i].first,ans[i].second);
	return 0;
}