P5087 Math.

On the magical land of `Xiaoben`, there is an infamous coach `Xiaoben`. Solution write-up: https://blog.csdn.net/kkkksc03/article/details/84928333.

`Xiaoben` loves multiplication. His favorite thing to do is: choose $K$ numbers from a sequence with $N$ elements (note: you cannot choose the same element multiple times, but choosing different elements with the same value is allowed), and then take the product of these $K$ numbers as the score of this combination. `Xiaoben` wants to try all such combinations and compute the sum of the scores of all combinations. But he also needs to run a mock contest to crush us newbies, so he has to hand this task to you. `Xiaoben` (~~in some ways~~) is still very kind, so you do not need to use big integers. You only need to output the answer modulo $10^9+7$.

The first line contains two integers $N$ and $K$. The second line contains $N$ integers $A_i$ describing the sequence.

Output one integer in one line, representing the answer.

#### Explanation for Sample #2: `Xiaoben` can choose four combinations: `{A[1],A[2],A[3]},{A[1],A[2],A[4]},{A[1],A[3],A[4]},{A[2],A[3],A[4]}`. Their scores are $1,2,2,2$ respectively. The total is $7$. #### Constraints: For $10\%$ of the testdata, $N\le 5000, K\le 2$. For $30\%$ of the testdata, $N\le 10^5, K\le 3$. For $50\%$ of the testdata, $N\le 10^5, K\le 5$. For $100\%$ of the testdata, $1\le N\le 10^5, 1\le K \le 300 \& \& K\le N, 1\le A[i]\le 10^8$. Translated by ChatGPT 5