#include <iostream>
#include <queue>
#include <bitset>
#include <cstdio>
using namespace std;
int a[4][4];
unsigned int flag[10000];
unsigned int step[10000];

unsigned int stat , temp;
queue<int>q;
int main()
{
   int flip(int i,int j, int stat);
   stat<<=1;
   for(int i=0; i<4;i++,getchar())
     for(int j=0;j<4;j++)
       {a[i][j]=getchar();
       stat<<=1;
       a[i][j]=='b'?stat++:1;
       }

  q.push(stat);
   step[stat]=0;
   flag[stat]=1;
   while (q.size())
     {
       stat=q.front();
       for(int i=0;i<4;i++)
          for(int j=0;j<4;j++)
            {
            temp =flip(i,j,stat);
            if(temp==0x0000||temp==0xffff) {cout<<step[stat]+1<<endl; return 0 ;}
            else  {
                  q.push(temp);
                  step[temp]=step[stat]+1;
                  flag[temp]=1;
                  }
            }

        q.pop();
     }
    cout<<"impossible"<<endl;
    return 0;

}
 int flip(int i,int j,   int stat)
{
    bitset<16>b(stat);
    b[i*4+j].flip();
	i>0 && b[(i-1)*4+j].flip();
	i<3 && b[(i+1)*4+j].flip();
	j>0 && b[i*4+(j-1)].flip();
	j<3 && b[i*4+(j+1)].flip();
    return b.to_ulong();
}