//入力チェック用
#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>
#include "assert.h"
#include <ctime>
#include <fstream>

using namespace std;

#define MAX_N 50
#define MIN_N 1


void checker(int n, int m, vector<string> &v){
	assert(MIN_N <= n);
	assert(n <= MAX_N);
	
	assert(MIN_N <= m);
	assert(m <= MAX_N);

	string s = "wb";
	assert(v.size() == n);
	for(int i=0; i<n; i++){
		assert(v[i].size() == m);
		for(int j=0; j<m; j++){
			assert( v[i][j] == s[(i+j)%2] || v[i][j] == '.');
		}
	}
}


int main(){
	int n,m;
	cin >> n >> m;
	vector<string> v(n);
	for(int i=0; i<n; i++){
		cin >> v[i];
	}
	checker(n,m,v);
	return 0;
}