/* Topcoder SRM 663 D2 250-pointer 
 * James S. Plank
 * November 11, 2016
 */

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

class ChessFloor {
  public:
    int minimumChanges(vector <string> floor);
};

int ChessFloor::minimumChanges(vector <string> floor)
{
  int i, j, b, w;
  int best, nc;

  /* Sentinelize best so that it is bigger than any legal answer. */

  best = floor.size() * floor.size() + 1;

  /* Enumerate all combinations of "black" and "white" */

  for (b = 'a'; b <= 'z'; b++) {
    for (w = 'a'; w <= 'z'; w++) {
      if (b != w) {

        /* For each of these, calculate the number of squares
           that you have to change to make it work. */

        nc = 0;
        for (i = 0; i < floor.size(); i++) {
          for (j = 0; j < floor.size(); j++) {
            if ((i+j)%2 == 0 && floor[i][j] != b) nc++;
            if ((i+j)%2 == 1 && floor[i][j] != w) nc++;
          }
        }

        /* Keep track of the minimum */

        if (nc < best) best = nc;
      }
    }
  }

  /* Return the minimum */

  return best;
}