#include <bits/stdc++.h>

using namespace std;

ifstream fin  ("barbar.in");
ofstream fout ("barbar.out");

const int INF = (1<<30);
const int MAXN = 1005;
int closestDragon[MAXN][MAXN], freq[MAXN][MAXN], best[MAXN][MAXN];
char mat[MAXN][MAXN]; string row;
int n, m, sol;

void debug(int test[MAXN][MAXN]){
    for(int i=1; i<=n; i++, cout<<"\n")
        for(int j=1; j<=m; j++, cout<<" ")
            if(mat[i][j] == '*')
                cout<<"*";
            else
                cout<<test[i][j];
}

struct End{
    int ii;
    int jj;
}in, out;

struct cell{
    int ii;
    int jj;
    int dd;
}aux, nod;
int di[] = {+1, -1,  0,  0};
int dj[] = { 0,  0, +1, -1};

void bfs1(int is, int js, int paint){

    queue <cell> q; aux.ii = is; aux.jj = js; aux.dd=0;
    q.push(aux);
    while(!q.empty()){
        nod = q.front();
        q.pop();

        closestDragon[nod.ii][nod.jj]=nod.dd;
        freq[nod.ii][nod.jj] = paint;

        for(int dir=0; dir < 4; dir++){
            int newi = nod.ii + di[dir];
            int newj = nod.jj + dj[dir];

            if(1 <= newi && newi <= n && 1 <= newj && newj <= m)
                if(mat[newi][newj] != '*')
                    if(freq[newi][newj] != paint)
                        if(closestDragon[newi][newj] > closestDragon[nod.ii][nod.jj] + 1){
                            aux.ii = newi;
                            aux.jj = newj;
                            aux.dd = closestDragon[nod.ii][nod.jj] + 1;
                            q.push(aux);
                        }
        }
    }
}

void bfs2(int is, int js, int paint){

    queue <cell> q;

    aux.ii = is; aux.jj = js; aux.dd=closestDragon[is][js];
    q.push(aux);
    best[is][js] = INF;

    while(!q.empty()){
        nod = q.front();
        q.pop();

        if(nod.ii == out.ii && nod.jj == out.jj){
            sol = max(sol, nod.dd);
            continue;
        }

        for(int dir=0; dir < 4; dir++){
            int newi = nod.ii + di[dir];
            int newj = nod.jj + dj[dir];

            if(1 <= newi && newi <= n && 1 <= newj && newj <= m)
                if(mat[newi][newj] != '*')
                    if(best[newi][newj] < nod.dd){

                        best[newi][newj] = nod.dd;
                        aux.ii = newi; aux.jj = newj; aux.dd = min(nod.dd, closestDragon[newi][newj]);
                        q.push(aux);
                    }
        }
    }
}

int main (){
    fin>>n>>m;
    for(int i=1; i<=n; i++){
        fin>>row;
        for(int j=0; j<m; j++){
            mat[i][j+1] = row[j];
            closestDragon[i][j+1] = INF;
            best[i][j+1] = -1;
        }
    }

    int color;
    for(int i=1; i<=n; i++)
        for(int j=1; j<=m; j++){
            if(mat[i][j] == 'D')
                bfs1(i, j, ++color);

            if(mat[i][j] == 'I'){
                in.ii=i;
                in.jj=j;
            }
            if(mat[i][j] == 'O'){
                out.ii=i;
                out.jj=j;
            }
        }

    //debug(closestDragon);

    sol=0;
    bfs2(in.ii, in.jj, ++color);
    fout<<sol;
    return 0;
}