#include <iostream>
#include <sstream>
#include <iomanip>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <algorithm>
#include <numeric>
#include <iterator>
#include <functional>
#include <complex>
#include <queue>
#include <stack>
#include <cmath>
#include <cassert>
using namespace std;
typedef long long LL;
typedef long double LD;
typedef complex<double> CMP;

static const int INF = 0x1fffffff;

int bitcnt(int x)
{
	int c = 0;
	for(; x; x>>=1)
		c += x&1;
	return c;
}

class YetAnotherBoardGame { public:
	int g_best;
	int minimumMoves(vector <string> board)
	{
		int H = board.size();
		int W = board[0].size();
		vector<int> b(H);
		for(int y=0; y<H; ++y)
			for(int x=0; x<W; ++x)
				b[y] |= (board[y][x]=='W')<<x;
		b.push_back(0); // sentinel

		g_best = INF;
		for(int type=1; type<=2; ++type)
			for(int mask=0; mask<(1<<W); ++mask)
			{
				int cur = b[0];
				int next = b[1];
				flip(cur, next, mask, type, W);
				do_try(cur, next, b, 1, H, W, (type==1 ? mask : 0), (type==2 ? mask : 0), bitcnt(mask));
			}
		return g_best==INF ? -1 : g_best;
	}

	void flip(int& cur, int& next, int mask, int type, int W)
	{
		int pat = (type==1 ? 5 : 7);
		for(int f=0; (1<<f)<=mask; ++f)
			if((1<<f) & mask)
				cur ^= (pat<<f>>1)&((1<<W)-1);
		next ^= mask;
	}

	void do_try(int prev, int cur, const vector<int>& b, int y, int H, int W, int must_t1, int must_t2, int acc)
	{
		if(y==H) {
			int score = (prev==0 ? 0 : INF) + acc;
			g_best = min(g_best, score);
			return;
		}

		int mask = prev;
		int score = bitcnt(mask) + acc;
		if(score >= g_best)
			return;

		if(!(mask&must_t2)) { // try type 1
			int curr = cur;
			int next = b[y+1];
			flip(curr, next, mask, 1, W);
			do_try(curr, next, b, y+1, H, W, must_t1|mask, must_t2, score);
		}
		if(!(mask&must_t1)) { // try type 2
			int curr = cur;
			int next = b[y+1];
			flip(curr, next, mask, 2, W);
			do_try(curr, next, b, y+1, H, W, must_t1, must_t2|mask, score);
		}
	}
};

// BEGIN CUT HERE
#include <ctime>
double start_time; string timer()
 { ostringstream os; os << " (" << int((clock()-start_time)/CLOCKS_PER_SEC*1000) << " msec)"; return os.str(); }
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v)
 { os << "{ ";
   for(typename vector<T>::const_iterator it=v.begin(); it!=v.end(); ++it)
   os << '\"' << *it << '\"' << (it+1==v.end() ? "" : ", "); os << " }"; return os; }
void verify_case(const int& Expected, const int& Received) {
 bool ok = (Expected == Received);
 if(ok) cerr << "PASSED" << timer() << endl;  else { cerr << "FAILED" << timer() << endl;
 cerr << "\to: \"" << Expected << '\"' << endl << "\tx: \"" << Received << '\"' << endl; } }
#define CASE(N) {cerr << "Test Case #" << N << "..." << flush; start_time=clock();
#define END	 verify_case(_, YetAnotherBoardGame().minimumMoves(board));}
int main(){

CASE(0)
	string board_[] = {"BBBBBBBBB",
 "BBWBBBBBB",
 "BWWWBBBBB",
 "BBWBBBWBB",
 "BBBBBWBWB",
 "BBBBBBWBB"};
	  vector <string> board(board_, board_+sizeof(board_)/sizeof(*board_)); 
	int _ = 2; 
END
CASE(1)
	string board_[] = {"BBW",
 "WWW",
 "BWW"};
	  vector <string> board(board_, board_+sizeof(board_)/sizeof(*board_)); 
	int _ = 2; 
END
CASE(2)
	string board_[] = {"WBW",
 "BBW",
 "WBW"};
	  vector <string> board(board_, board_+sizeof(board_)/sizeof(*board_)); 
	int _ = 4; 
END
CASE(3)
	string board_[] = {"BBBB",
 "WBWB",
 "BBBB",
 "BBBB"};
	  vector <string> board(board_, board_+sizeof(board_)/sizeof(*board_)); 
	int _ = -1; 
END
CASE(4)
	string board_[] = {"WWWWWBW",
 "WBWBWBW",
 "BBWBBWW"};
	  vector <string> board(board_, board_+sizeof(board_)/sizeof(*board_)); 
	int _ = 7; 
END
CASE(5)
	string board_[] = {"WWWWWWWWWW",
 "WWWWWWWWWW",
 "WWWWWWWWWW",
 "WWWWWWWWWW",
 "WWWWWWWWWW",
 "WWWWWWWWWW",
 "WWWWWWWWWW",
 "WWWWWWWWWW",
 "WWWWWWWWWW",
 "WWWWWWWWWW"}
;
	  vector <string> board(board_, board_+sizeof(board_)/sizeof(*board_)); 
	int _ = 30; 
END
/*
CASE(6)
	string board_[] = ;
	  vector <string> board(board_, board_+sizeof(board_)/sizeof(*board_)); 
	int _ = ; 
END
CASE(7)
	string board_[] = ;
	  vector <string> board(board_, board_+sizeof(board_)/sizeof(*board_)); 
	int _ = ; 
END
*/
}
// END CUT HERE
