1753

#include <iostream>
#include <queue>

using namespace std;

const int N = 4;
bool State[65536];
bool Find = false;
int dir[4][2] = {{-1, 0}, {0, 1}, {1, 0}, {0, -1}};

struct rec
{
    bool map[N][N];
    int step;
};

rec start;
queue<rec> q;

int GetSate(bool map[][N])
{
    int sum = 0;
    for(int i = 0; i < N; i++)
    {
        for(int j = 0; j < N; j++)
            if(map[i][j])
    sum += 1;
            else
    sum <<= 1;
    }
    return sum;
}

void Change(rec& r, int i, int j)
{
 int k;
 int x;
 int y;

 r.map[i][j] = !r.map[i][j];
    for(k = 0; k < N; k++)
    {
        x = i+dir[k][0];
  y = j+dir[k][1];
        if(x >= 0 && x < N && y >= 0 && y < N)
        {
   r.map[x][y] = !r.map[x][y];
        }
    }
}

void Init()
{
    char ch;
    for(int i = 0; i < 65536; i++)
  State[i] = true;

    for(int i = 0; i < N; i++)
        for(int j = 0; j < N; j++)
        {
            cin >> ch;
            if(ch == 'b')
    start.map[i][j] = true;
   else
    start.map[i][j] = false;
        }
    start.step = 0;
    q.push(start);
 State[GetSate(start.map)] = false;
}

bool Check(bool map[N][N])
{
    bool Stand = map[0][0];
    for(int i = 0; i < N; i++)
        for(int j = 0; j < N; j++)
            if(map[i][j] != Stand)
    return false;
    return true;
}

void BFS()
{
 rec r, t;
 int i;
 int j;
 int temp;
 if(!q.empty())
    {
        r = q.front();
        q.pop();
  if(Check(r.map))
        {
   cout << r.step << endl;
            Find = true;
            return;
        }
        else
        {
            for(i = 0; i < N; i++)
                for(j = 0; j < N; j++)
                {
                    t = r;
                    Change(t, i, j);
     temp = GetSate(t.map);
                    if(State[temp])
                    {
                        t.step += 1;
                        q.push(t);
                        State[temp] = false;
                    }
                }
        }
        BFS();
    }
}

int main()
{
    Init();
    BFS();
    if(!Find)
  cout << "Impossible" << endl;
    return 0;
}