> #include<iostream>
> #include<cmath>
> using namespace std;
> 
> #define N 4
> 
> int minCount = 100000;
> int map[N][N];
> 
> bool isFinish();
> 
> void output(){
> 	int i,j;
> 	for(i=0;i<N;i++){
> 		for(j=0;j<N;j++){
> 			cout<<map[i][j];
> 		}
> 		cout<<endl;
> 	}
> }
> 
> void roll(int i){
> 	int column,row,pianyi_x,pianyi_y;
>     column = i % N;
> 	row = i / N;
> //	system("pause");
> //	printf("(%d,%d)\n",row, column);
> //	cout<<"("<<i<<")"<<endl;
> 	pianyi_x = row;
> 	pianyi_y = column ;
> 	if(pianyi_x>=0 && pianyi_x<N && pianyi_y>=0 && pianyi_y<N){
> 		map[pianyi_x][pianyi_y] = ~map[pianyi_x][pianyi_y];
> 	}
> 
> 
> 	pianyi_x = row - 1;
> 	pianyi_y = column ;
> 	if(pianyi_x>=0 && pianyi_x<N && pianyi_y>=0 && pianyi_y<N){
> 		map[pianyi_x][pianyi_y] = ~map[pianyi_x][pianyi_y];
> 	}
> 
> 	pianyi_x = row + 1;
> 	pianyi_y = column;
> 	if(pianyi_x>=0 && pianyi_x<N && pianyi_y>=0 && pianyi_y<N){
> 		map[pianyi_x][pianyi_y] = ~map[pianyi_x][pianyi_y];
> 	}
> 
>     pianyi_x = row ;
> 	pianyi_y = column - 1;
> 	if(pianyi_x>=0 && pianyi_x<N && pianyi_y>=0 && pianyi_y<N){
> 		map[pianyi_x][pianyi_y] = ~map[pianyi_x][pianyi_y];
> 	}
> 
>     pianyi_x = row ;
> 	pianyi_y = column + 1;
> 	if(pianyi_x>=0 && pianyi_x<N && pianyi_y>=0 && pianyi_y<N){
> 		map[pianyi_x][pianyi_y] = ~map[pianyi_x][pianyi_y];
> 	}
> 		
> //	output();
> }
> 	
> int changeOne(int start,int cnt){
> 	int i,column,row,pianyi_x,pianyi_y;
> 	for(i=start; i< N*N; i++){
> 		roll(i);
> 		cnt++;
> 		if(isFinish()==true){
> 			if(minCount>cnt) minCount = cnt;
> 		}
> 		else{
> 			cnt = changeOne(i+1,cnt);
> 		}
> 		roll(i);
> 		cnt--;
>  	}
> 	return cnt;
> }
> 	
> bool isFinish(){
> 	int i,j;
> 	for(i=0; i<N; ++i){
> 		for(j=0; j<N; ++j){
> 			if( map[i][j]==0 ) return false;
> 		}
> 	}
> 	return true;
> }
> 
> int main()
> {
> 	int i,j;
> 	char aChar;
> 	for(i = 0;i < N ; i++ ){
> 		
> 		for(j = 0 ; j < N ; j++){
>             cin>>aChar;
> 			if( aChar=='b'){
> 				map[i][j] = -1;
> 			}
> 			else {
> 				map[i][j] = 0;
> 			}
> 		}
> 	}//input over
> //	output();
> 	if( isFinish()== true)
> 	    printf("0\n");
> 	else{
> //		cout<<changeOne(0,0)<<endl;
> 		changeOne(0,0);
> 		if(minCount!=100000) printf("%d",minCount);
> 		else printf("Impossible");
> 	}
> 	system("pause");
> 	return 0;
> }