> #include<iostream>
> #include<cstdio>
> using namespace std;
> int d[1000000],w[1000000],p[1000000];
> int main()
> {
>     int i,j,k=0,g=0,h=1;
>     for(i=1;i<=4;i++){
>         for(j=1;j<=4;j++)
>         if (getchar()=='b') d[1]=(d[1]<<1)+1; else d[1]=d[1]<<1;
>         getchar();
>     }
>     p[1]=0;
>     while(g<h){
>         g++;
>         if(d[g]==0||d[g]==(1<<16)-1) {
>             printf("%d",w[g]);
>             k=1;
>             break;
>         }
>         if (h>990000) continue;
>         for(i=p[g]+1;i<=16;i++){
>             h++;
>             w[h]=w[g]+1;
>             p[h]=i;
>             d[h]=d[g]^(1<<(16-i));
>             if(i-4>=1) d[h]^=1<<(16-i+4);
>             if(i+4<=16) d[h]^=1<<(16-i-4);
>             if(i%4!=0) d[h]^=1<<(16-i-1);
>             if((i-1)%4!=0) d[h]^=1<<(16-i+1);
>         }
>     }
>     if (!k) printf("Impossible");
>     return 0;
> }