#include <stdio.h>

FILE *in;
int n, p1, p2, p3;
char m[128][128];
int r[128][128][128], depp;

void order(int &p1, int &p2, int &p3)
{
  int m;
  if (p1>p2)
    {
      m=p1; p1=p2; p2=m;
    }
  if (p1>p3)
    {
      m=p1; p1=p3; p3=m;
    }
  if (p2>p3)
    {
      m=p2; p2=p3; p3=m;
    }
}

int main()
{
  in = fopen("hike.in", "r");

  while(1)
    {
      fscanf(in, " %d ", &n);
      if (n==0) break;
      fscanf(in, " %d %d %d ", &p1, &p2, &p3);
      for(int i=0; i<n; i++)
	{
	  for(int j=0; j<n; j++)
	    {
	      fscanf(in, " %c ", &m[i][j]); 
	      for(int k=0; k<n; k++)
		{
		  r[i][j][k] = -1;
		}
	    }
	  r[i][i][i] = 0;
	}
      p1--; p2--; p3--;
      order(p1, p2, p3);

      int cnt = 0;
      depp = 1;
      while(depp)
	{
	  if (r[p1][p2][p3] > -1 && r[p1][p2][p3] <= cnt) break;
          ++cnt;
	  depp = 0;
	  for(int i=0; i<n; i++)
	    {
	      for(int j=i; j<n; j++)
		{
		  for(int k=j; k<n; k++)
		    {
		      if (r[i][j][k] != -1)
			{
			  for(int l=0; l<n; l++)
			    {
			      if (m[l][i] == m[j][k])
				{
				  int a1=l;
				  int a2=j;
				  int a3=k;
				  order(a1,a2,a3);
				  if ((r[a1][a2][a3] == -1) || (r[a1][a2][a3] > r[i][j][k]+1))
				    {
				      r[a1][a2][a3] = r[i][j][k]+1;
				      depp = 1;
				    }
				}
			      if (m[l][j] == m[i][k])
				{
				  int a1=l;
				  int a2=i;
				  int a3=k;
				  order(a1,a2,a3);
				  if ((r[a1][a2][a3] == -1) || (r[a1][a2][a3] > r[i][j][k]+1))
				    {
				      r[a1][a2][a3] = r[i][j][k]+1;
				      depp = 1;
				    }
				}
			      if (m[l][k] == m[i][j])
				{
				  int a1=l;
				  int a2=i;
				  int a3=j;
				  order(a1,a2,a3);
				  if ((r[a1][a2][a3] == -1) || (r[a1][a2][a3] > r[i][j][k]+1))
				    {
				      r[a1][a2][a3] = r[i][j][k]+1;
				      depp = 1;
				    }
				}
			    }
			}
		    }
		}
	    }
	}

      if (r[p1][p2][p3] == -1)
	{
	  printf("impossible\n");
	}
      else
	{
	  printf("%d\n", r[p1][p2][p3]);
	}
    }

  return 0;
}