#include <bits/stdc++.h>
using namespace std;
long long n, k;
long long mas = 0;
vector<pair<long long, long long>> ans(1000000);
void solve(vector<vector<long long>> poses) {
  vector<long long> all;
  for (long long i = 0; i < poses.size(); i++) {
    for (long long j = 0; j < k; j++) {
      all.push_back(poses[i][j]);
    }
  }
  sort(all.begin(), all.end());
  map<long long, long long> conv;
  for (long long i = 0; i < all.size(); i++) {
    conv[all[i]] = i;
  }
  long long len = all.size();
  for (long long i = 0; i < poses.size(); i++) {
    for (long long j = 0; j < k; j++) {
      poses[i][j] = conv[poses[i][j]];
    }
  }
  vector<long long> start(poses.size());
  vector<bool> used(poses.size());
  for (long long _ = 0; _ < poses.size(); _++) {
    long long val = 1000000;
    long long ind = -1;
    for (long long i = 0; i < poses.size(); i++) {
      if (used[i]) {
        continue;
      }
      if (poses[i][start[i] + 1] < val) {
        val = poses[i][start[i] + 1];
        ind = i;
      }
    }
    ans[mas + ind].first = all[poses[ind][start[ind]]] + 1;
    ans[mas + ind].second = all[poses[ind][start[ind] + 1]] + 1;
    used[ind] = true;
    for (long long i = 0; i < poses.size(); i++) {
      if (used[i]) {
        continue;
      }
      if (poses[i][start[i]] < poses[ind][start[ind] + 1]) {
        start[i]++;
      }
    }
  }
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> k;
  vector<vector<long long>> poses(n);
  for (long long i = 0; i < n * k; i++) {
    long long c;
    cin >> c;
    poses[c - 1].push_back(i);
  }
  for (long long i = 0; i < n; i += k - 1) {
    vector<vector<long long>> kek;
    for (long long j = i; j < min(i + k - 1, n); j++) {
      kek.push_back(poses[j]);
    }
    mas = i;
    solve(kek);
  }
  for (long long i = 0; i < n; i++) {
    cout << ans[i].first << " " << ans[i].second << '\n';
  }
}
