write a go solution for 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 ceil(n/k-1) intervals.

One can show that such a family of intervals always exists under the given constraints.

Input Format:
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 Format:
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.

Note:
In the first sample, each number can be contained in at most ceil(4/3-1)=2 intervals. The output is described by the following picture:

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 ceil(1/2-1)=1 interval.

In the third sample, each number can be contained in at most ceil(3/3-1)=2 intervals. The output is described by the following picture:. Output only the code with no comments, explanation, or additional text.