/* Solution to Topcoder SRM 729, D2, 250-pointer.
   BrokenChessboard
   James S. Plank
   Fri Sep  7 14:00:20 EDT 2018
 */

#include <string>
#include <vector>
#include <iostream>
using namespace std;

class BrokenChessboard {
  public:
    int minimumFixes(vector <string> board);
};

int BrokenChessboard::minimumFixes(vector <string> board)
{
  int r, c;
  int SW, SB;

  SW = 0; 
  SB = 0;

  /* Loop through every square of the checkerboard. */

  for (r = 0; r < board.size(); r++) {
    for (c = 0; c < board[r].size(); c++) {

      /* These squares will match the (0,0) square.  
         So, if they are 'B', you need to add one to SW.
         Otherwise, you add one to SB. */

      if ((r + c) %2 == 0) {
        if (board[r][c] == 'B') SW++; else SB++;

      /* These squares will match the (0,1) square.  
         So, if they are 'B', you need to add one to SB.
         Otherwise, you add one to SW. */

      } else {
        if (board[r][c] == 'W') SW++; else SB++;
      }
    }
  }

  /* Return the smaller of the two sums. */

  return (SW < SB) ? SW : SB;
}