write a go solution for Description:
Mr. Chanek opened a letter from his fellow, who is currently studying at Singanesia. Here is what it says.

Define an array b (0<=b_i<k) with n integers. While there exists a pair (i,j) such that b_ineb_j, do the following operation:

- Randomly pick a number i satisfying 0<=i<n. Note that each number i has a probability of 1/n to be picked.
- Randomly Pick a number j satisfying 0<=j<k.
- Change the value of b_i to j. It is possible for b_i to be changed to the same value.

Denote f(b) as the expected number of operations done to b until all elements of b are equal.

You are given two integers n and k, and an array a (-1<=a_i<k) of n integers.

For every index i with a_i=-1, replace a_i with a random number j satisfying 0<=j<k. Let c be the number of occurrences of -1 in a. There are k^c possibilites of a after the replacement, each with equal probability of being the final array.

Find the expected value of f(a) modulo 10^9+7.

Formally, let M=10^9+7. It can be shown that the answer can be expressed as an irreducible fraction p/q, where p and q are integers and qnotequiv0pmodM. Output the integer equal to p*q^-1bmodM. In other words, output such an integer x that 0<=x<M and x*qequivppmodM.

After reading the letter, Mr. Chanek gave the task to you. Solve it for the sake of their friendship!

Input Format:
The first line contains two integers n and k (2<=n<=10^5, 2<=k<=10^9).

The second line contains n integers a_1,a_2,ldots,a_n (-1<=a_i<k).

Output Format:
Output an integer denoting the expected value of f(a) modulo 10^9+7.

Note:
None. Output only the code with no comments, explanation, or additional text.