description: The numbers 1,   2,   ...,   n ⋅ k are colored with n colors. These colors are indexed by 1,   2,   ...,   n. For each 1 ≤ i ≤ n, there are exactly k numbers colored with color i.
  
  Let [a,   b] denote the interval of integers between a and b inclusive, that is, the set \\{a,   a + 1,   ...,   b\}. You must choose n intervals [a_1,   b_1],   [a_2,   b_2],   ..., [a_n,   b_n] such that: 
  
    * for each 1 ≤ i ≤ n, it holds 1 ≤ a_i < b_i ≤ n ⋅ k; 
    * for each 1 ≤ i ≤ n, the numbers a_i and b_i are colored with color i; 
    * each number 1 ≤ x ≤ n ⋅ k belongs to at most \left⌈ (n)/(k - 1) \right⌉ intervals. 
  
  
  
  One can show that such a family of intervals always exists under the given constraints.
  
  Input
  
  The first line contains two integers n and k (1 ≤ n ≤ 100, 2 ≤ k ≤ 100) — the number of colors and the number of occurrences of each color.
  
  The second line contains n ⋅ k integers c_1,   c_2,   ...,   c_{nk} (1 ≤ c_j ≤ n), where c_j is the color of number j. It is guaranteed that, for each 1 ≤ i ≤ n, it holds c_j = i for exactly k distinct indices j.
  
  Output
  
  Output n lines. The i-th line should contain the two integers a_i and b_i.
  
  If there are multiple valid choices of the intervals, output any.
  
  Examples
  
  Input
  
  
  4 3
  2 4 3 1 1 4 2 3 2 1 3 4
  
  
  Output
  
  
  4 5
  1 7
  8 11
  6 12
  
  Input
  
  
  1 2
  1 1
  
  
  Output
  
  
  1 2
  
  
  Input
  
  
  3 3
  3 1 2 3 2 1 2 1 3
  
  
  Output
  
  
  6 8
  3 7
  1 4
  
  Input
  
  
  2 3
  2 1 1 1 2 2
  
  
  Output
  
  
  2 3
  5 6
  
  Note
  
  In the first sample, each number can be contained in at most \left⌈ (4)/(3 - 1) \right⌉ = 2 intervals. The output is described by the following picture:
  
  <image>
  
  In the second sample, the only interval to be chosen is forced to be [1,   2], and each number is indeed contained in at most \left⌈ (1)/(2 - 1) \right⌉ = 1 interval.
  
  In the third sample, each number can be contained in at most \left⌈ (3)/(3 - 1) \right⌉ = 2 intervals. The output is described by the following picture:
  
  <image>
solution: #include <bits/stdc++.h>
  using namespace std;
  using ll = long long;
  using pii = pair<int, int>;
  using vi = vector<int>;
  int main() {
    cin.tie(0)->sync_with_stdio(0);
    cin.exceptions(cin.failbit);
    int n, k;
    cin >> n >> k;
    vector<int> a(n * k);
    vector<vi> occ(n);
    for (int i = 0; i < n * k; ++i) {
      cin >> a[i];
      --a[i];
      occ[a[i]].push_back(i);
    }
    int goal = (n + k - 2) / (k - 1);
    int l = 0;
    vi freq(n, 0);
    vi done(n, 0);
    vi ready;
    vector<pii> ans(n, {-1, -1});
    for (int r = 0; r < n * k; ++r) {
      if (!done[a[r]] && ++freq[a[r]] == 2) {
        ready.push_back(a[r]);
      }
      if (ready.size() == goal) {
        for (int x : ready) {
          auto it = upper_bound(begin(occ[x]), end(occ[x]), r);
          assert(it - begin(occ[x]) >= 2);
          ans[x].second = *prev(it);
          ans[x].first = *prev(prev(it));
          assert(l <= ans[x].first && ans[x].first < ans[x].second &&
                 ans[x].second <= r);
          done[x] = true;
        }
        freq.assign(n, 0);
        ready.clear();
        l = r + 1;
      }
    }
    for (int i = 0; i < n; ++i) {
      if (!done[i]) {
        int m = (int)(occ[i]).size();
        ans[i].first = occ[i][m - 2];
        ans[i].second = occ[i][m - 1];
      }
    }
    for (int i = 0; i < n; ++i) {
      cout << ans[i].first + 1 << " " << ans[i].second + 1 << '\n';
    }
    return 0;
  }
  