Find Min Num Moves

Each operation flips one B/W cell. Make every row and every column read the same forwards and backwards, and find the minimum number of flips. The input uses uppercase B/W throughout.

输入格式

This site's ranges: the first line contains N M (1..500), followed by N lines each with M characters B/W.

输出格式

Output a single integer, the answer.

样例 1

输入：

1 2
BW

输出：

1

样例 2

输入：

2 2
BW
WB

输出：

2

样例 3

输入：

3 3
BWB
WWW
BWB

输出：

0

样例说明

Example 1: the two cells must match, so flip either one; the answer is 1.

Example 2: the four corners belong to the same group, with two B and two W, so at least 2 cells must be flipped.

Example 3: every row and column is already a palindrome, so the answer is 0.