The Lalalandia's Christmas is coming! Only N days are left and Jambo wants to buy all his friends a present. There are N presents that his friends want and Jambo, because he is a very small elephant, cannot buy and carry more than K presents at any day. Every present in jth day costs $a_i * j$ for every $1 \le j \le N$. Can you help Jambo determine the minimum amount of money needed to buy all presents and make his friends happy?

Input format

The first line consists of integers N - the number of days and the number of presents Jambo wants to buy, and K - the number of presents Jambo can carry. The second line contains n integers $a_1, a_2, ..., a_N$ — the cost of the ith present.

Output format

The first line should contain the single integer - answer to the problem.

Constraints:

$1 \le N \le 10^5$

$1 \le K \le N$

$1 \le a_i \le 10^9$