题目描述

Pixels in a digital picture can be represented with three integers in the range $0$ to $255$ that indicate the intensity of the red, green, and blue colors. To compress an image or to create an artistic effect, many photo-editing tools include a posterize operation which works as follows. Each color channel is examined separately; this problem focuses only on the red channel. Rather than allow all integers from $0$ to $255$ for the red channel, a posterized image allows at most $k$ integers from this range. Each pixel's original red intensity is replaced with the nearest of the allowed integers. The photo-editing tool selects a set of $k$ integers that minimizes the sum of the squared errors introduced across all pixels in the original image. If there are $n$ pixels that have original red values $r_{1},$ . . . , $r_{n},$ and $k$ allowed integers $v_{1},$ . . . , $v_{k},$ the sum of squared errors is defined as

$\(\su_m_{i=1}^{n}{\mi_n_{1 \le j \le k}{(r_i - v_j)^2}}\).$

Your task is to compute the minimum achievable sum of squared errors, given parameter $k$ and a description of the red intensities of an image's pixels.

输入格式

The first line of the input contains two integers $d (1 \le d \le 256)$ , the number of distinct red values that occur in the original image, and $k (1 \le k \le d)$ , the number of distinct red values allowed in the posterized image. The remaining $d$ lines indicate the number of pixels of the image having various red values. Each such line contains two integers $r (0 \le r \le 255)$ and $p (1 \le p \le 2^{26}),$ where $r$ is a red intensity value and $p$ is the number of pixels having red intensity $r$ . Those $d$ lines are given in increasing order of red value.

输出格式

Display the sum of the squared errors for an optimally chosen set of $k$ allowed integer values.

题目大意

给定 $d$ 类元素，第 $i$ 类元素的取值为 $r_i$，且有 $p_i$ 个，按照这些信息可以将这些元素排列在一个长度为 $n=\sum p_i$ 的序列里，现在你要做的是规划一个长度为 $k$ 的序列 $v_i$，使得按照如下定义的序列误差最小：

$$\sum\limits_{i=1}^n \min\limits_{1 \le j \le k}(r_i-v_j)^2 $$

求最小序列误差。

$1 \le k \le d \le 256$，$0 \le r \le 255$，$1 \le p \le 2^{26}$。

翻译者：一只书虫仔

2 1
50 20000
150 10000

66670000

2 2
50 20000
150 10000

0

4 2
0 30000
25 30000
50 30000
255 30000

37500000

提示

Time limit: 2 s, Memory limit: 512 MB.