CTFLAG  Flag
According to a new ISO standard, a flag of every country should have, strangely enough, a chequered field n × m, each square should be wholly painted one of 26 colours. The following restrictions are set:
 In each row at most two different colours can be used.
 No two adjacent squares can be painted the same colour.
Pay attention, please, that in one column more than two different colours can be used.
Berland's government took a decision to introduce changes into their country's flag in accordance with the new standard, at the same time they want these changes to be minimal. By the given description of Berland's flag you should find out the minimum amount of squares that need to be painted different colour to make the flag meet the new ISO standard. You are as well to build one of the possible variants of the new Berland's flag.
Input
The first input line contains 2 integers n and m (1 ≤ n, m ≤ 500) — number of rows and columns in Berland's flag respectively. Then there follows the flag's description: each of the following n lines contains m characters. Each character is a letter from a to z, and it stands for the colour of the corresponding square.
Output
Output the minimum number of squares that need to be repainted to make the flag meet the new ISO standard.
Example
Input: 3 4 aaaa bbbb cccc Output: 6
hide comments
anonymous:
20160422 17:45:32
Note: You are only required to find the minimum number of squares that have to


lite_coder:
20140728 07:12:20
nyc problem... 

Jashan Goyal:
20140722 01:24:04
Poorly managed for Python.... 
Added by:  Phan Công Minh 
Date:  20100621 
Time limit:  0.100s1s 
Source limit:  50000B 
Memory limit:  1536MB 
Cluster:  Cube (Intel G860) 
Languages:  All except: NODEJS OBJC PERL6 SQLITE VB.NET 
Resource:  Codeforces 