#include <fstream>
#include <queue>

using namespace std;

const int LMAX = 1000;
const int CMAX = 1000;

int l, c;
int temnita[2 + LMAX][2 + CMAX];
int viz[2 + LMAX][2 + CMAX];
bool obs[2 + LMAX][2 + CMAX];

pair<int, int> start, stop;

queue<pair<int, int> > q;

int dx[] = {1, 0, -1,  0};
int dy[] = {0, 1,  0, -1};

void fill()
{
  while (!q.empty())
  {
    int x = q.front().first;
    int y = q.front().second;
    q.pop();

    for(int i = 0; i < 4; i++)
    {
      int x_crt = x + dx[i];
      int y_crt = y + dy[i];
      if(!obs[x_crt][y_crt] && temnita[x_crt][y_crt] == 0)
      {
        temnita[x_crt][y_crt] = temnita[x][y] + 1;
        q.push(make_pair(x_crt, y_crt));
      }
    }
  }
}

bool bfs(int maxim)
{
   while(!q.empty())
   {
     q.pop();
   }

   viz[start.first][start.second] = maxim;

   q.push(start);
   while(!q.empty())
   {
     int x = q.front().first;
     int y = q.front().second;
     q.pop();
     for(int i = 0; i < 4; i++)
     {
       int x_crt = x + dx[i];
       int y_crt = y + dy[i];
       if (!obs[x_crt][y_crt] && viz[x_crt][y_crt] != maxim && temnita[x_crt][y_crt] >= maxim)
       {
         viz[x_crt][y_crt] = maxim;
         q.push(make_pair(x_crt, y_crt));
         if(x_crt == stop.first && y_crt == stop.second)
         {
           return true;
         }
       }
     }
   }

   return false;
}

int main()
{
  ifstream in("barbar.in");
  ofstream out("barbar.out");

  char ch;
  in >> l >> c;

  for (int i = 1; i <= l; i++)
  {
    for (int j = 1; j <= c; j++)
    {
      in >> ch;
      if (ch == 'I')
      {
        start.first = i;
        start.second = j;
      }
      else if (ch == '*')
      {
        obs[i][j] = true;
      }
      else if (ch == 'O')
      {
        stop.first = i;
        stop.second = j;
      }
      else if (ch == 'D')
      {
        obs[i][j] = true;
        q.push(make_pair(i, j));
      }
    }
  }

  for(int i = 0; i <= l + 1; i++)
  {
    obs[i][0]     = true;
    obs[i][c + 1] = true;
  }

  for(int j = 0; j <= c + 1; j++)
  {
    obs[0][j]     = true;
    obs[l + 1][j] = true;
  }

  fill();

  int st = 1, dr = temnita[start.first][start.second], last = - 1;

  while(st <= dr)
  {
    int mij = (st + dr)/2;
    if(bfs(mij))
    {
      st = mij + 1;
      last = mij;
    }
    else
    {
      dr = mij - 1;
    }
  }

  out << last;

  /*
  for(int i = 1; i <= l; i++)
  {
    for(int j = 1; j <= c; j++)
    {
      out << temnita[i][j] << ' ';
    }
    out << '\n';
  }
  */

  return 0;
}