//A tree without skin will surely die.
//A man without face is invincible.
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#include <bits/stdc++.h>

#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(S) ((int)S.size())
#define FOR(i, st_, n) for(int i = st_; i < n; ++i)
#define RFOR(i, n, end_) for(int i = (n)-1; i >= end_; --i)
#define x first
#define y second
#define pb push_back
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> pll;
typedef pair<int, int> pii;
typedef pair<double, double> pdd;
typedef unsigned long long ull;
typedef long double LD;
typedef pair<ull, ull> pull;
using namespace __gnu_pbds;
typedef tree<ll, null_type, less<>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;
using namespace std;
#ifdef ONPC
mt19937 rnd(228);
#else
mt19937 rnd(chrono::high_resolution_clock::now().time_since_epoch().count());
#endif


int solve()
{
	int n, m;
	if(!(cin>>n>>m))return 1;
	vector<string> a(n);
	FOR(i,0,n)cin>>a[i];
	
	auto is_ok=[&](int x, int y) -> bool{
		return x==0 || x==n-1 || y==0 || y==m-1 || a[x-1][y]=='-' || a[x+1][y]=='-' || a[x][y-1]=='-' || a[x][y+1]=='-';
	};
	set<pii> Q;
	FOR(i,0,n)
	{
		FOR(j,0,m)
		{
			if(a[i][j] == 'X' && is_ok(i,j))
			{
				Q.insert({i,j});
			}
		}
	}
	int res=0;
	while(!Q.empty())
	{
		++res;
		set<pii> nxt;
		for(auto [x,y]:Q)
			a[x][y]='-';
		for(auto [x,y]:Q)
		{
			if(x-1>=0 && a[x-1][y]=='X')
				nxt.insert({x-1,y});
			if(x+1<n && a[x+1][y]=='X')
				nxt.insert({x+1,y});
			if(y-1>=0 && a[x][y-1]=='X')
				nxt.insert({x,y-1});
			if(y+1>=0 && a[x][y+1]=='X')
				nxt.insert({x,y+1});
		}
		Q=move(nxt);
	}
	cout<<res<<'\n';
	return 0;
}

int32_t main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    int TET = 1e9;
    //cin >> TET;
    for (int i = 1; i <= TET; i++)
    {
        if (solve())
        {
            break;
        }
#ifdef ONPC
        cout << "__________________________" << endl;
#endif
    }
#ifdef ONPC
    cerr << endl << "finished in " << clock() * 1.0 / CLOCKS_PER_SEC << " sec" << endl;
#endif
}