> #include<iostream>
> #include<stdio.h>
> #include<deque>
> #define N 65533+3
> using namespace std;
> int a[N];
> int c[16][4]={
>     {1,4,-1,-1},
>     {0,2,5,-1},
>     {1,6,3,-1},
>     {2,7,-1,-1},
> 
>     {0,5,8,-1},
>     {1,4,6,9},
>     {2,5,7,10},
>     {3,6,11,-1},
> 
>     {4,9,12,-1},
>     {5,8,10,13},
>     {6,9,11,14},
>     {7,10,15,-1},
> 
>     {8,13,-1,-1},
>     {9,12,14,-1},
>     {13,10,15,-1},
>     {11,14,-1,-1}
> };
> 
> unsigned int get_start(int *A)
> {
>     unsigned int x=0;
>     for(int i=0;i<16;i++)
>     {
>         if(A[i])
>         {
>             x|=(1<<i);
>         }
>     }
>     return x;
> 
> }
> 
> 
> int start[16];
> int main()
> {
>     //freopen("/home/zlq/easy_acming/in","r",stdin);
>     for(int i=0;i<N;i++){a[i]=-1;}//全部设置成-1;
>     string tmp;
>     for(int line=0;line<4;line++)
>     {
>         cin>>tmp;
>         for(int i=0;i<4;i++)
>         {
>             if(tmp[i]=='b')
>             {
>                 start[line*4+i]=1;
>             }
>         }
>     }
>     int begin=get_start(start);
>     a[begin]=0;
>     deque<unsigned int> d;
>     d.push_back(begin);
>     int now;
>     int flag=0;
>     while(d.empty()==false)
>     {
>         now=d.front();
>         d.pop_front();
>         if(now==65535||now==0){flag=1;break;}
>         else
>         {
>             for(int i=0;i<16;i++)//一共可以有16种选择..16中变换
>             {
>                 unsigned int gen=now;
>                 gen=gen^(1<<i);
>                 for(int j=0;j<4;j++)//每种变换,将其中的4位进行取反操作
>                 {
>                     if(c[i][j]!=-1)
>                     gen=gen^(1<<c[i][j]);
>                 }
>                 if(a[gen]==-1)
>                 {
>                     a[gen]=a[now]+1;
>                     d.push_back(gen);
>                 }
> 
>             }
>         }
>     }
>     if(flag==1)
>     cout<<a[now]<<endl;
>     else
>     cout<<"Impossible"<<endl;
> 
> }