using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text;
using System.Globalization;
using System.Diagnostics;
using static System.Console;
//using System.Numerics;
//using static System.Math;
class Program
{
static void Main()
{
//SetOut(new StreamWriter(OpenStandardOutput()) { AutoFlush = false });
new Program().solve();
Out.Flush();
}
Scanner cin = new Scanner();
readonly int[] dd = { 0, 1, 0, -1, 0 }; //→↓←↑
readonly int mod = 1000000007;
int H, W;
void solve()
{
H = cin.nextint;
W = cin.nextint;
var S = new char[H][];
for (int i = 0; i < H; i++)
{
S[i] = cin.next.ToCharArray();
}
int T = 2 * H * W + 1;
var D = new Ford_Fulkerson(T + 1);
for (int i = 0; i < H; i++)
{
for (int j = 0; j < W; j++)
{
if (i == 0 || i == H - 1 || j == 0 || j == W - 1)
{
if (S[i][j] == 'X')
{
WriteLine(-1);
return;
}
D.add_edge(0, IN(i, j), Ford_Fulkerson.INF);
}
if (S[i][j] == 'X')
{
D.add_edge(IN(i, j), OUT(i, j), Ford_Fulkerson.INF);
D.add_edge(OUT(i, j), T, Ford_Fulkerson.INF);
}
else
{
D.add_edge(IN(i, j), OUT(i, j), 1);
for (int r = 0; r < 4; r++)
{
int y = i + dd[r];
int x = j + dd[r + 1];
if (y < 0 || y >= H || x < 0 || x >= W) continue;
D.add_edge(OUT(i, j), IN(y, x), Ford_Fulkerson.INF);
}
}
}
}
WriteLine(D.max_flow(0, T));
}
int IN(int y, int x)
{
return y * W + x + 1;
}
int OUT(int y, int x)
{
return y * W + x + 1 + H * W;
}
}
class Ford_Fulkerson
{
// 辺を表すクラス（行き先、容量、逆辺）
class Edge_
{
public int to;
public long cap;
public int rev;
public Edge_(int to, long cap, int rev)
{
this.to = to;
this.cap = cap;
this.rev = rev;
}
}
public const long INF = long.MaxValue / 3;
List<Edge_>[] G; // グラフの隣接リスト表現
bool[] used; // DFSですでに調べたかのフラグ
public Ford_Fulkerson(int V)
{
G = new List<Edge_>[V];
for (int i = 0; i < V; i++) G[i] = new List<Edge_>();
used = new bool[V];
}
// fromからtoまでの最短距離をBFSで計算する
public void add_edge(int from, int to, long cap)
{
G[from].Add(new Edge_(to, cap, G[to].Count));
G[to].Add(new Edge_(from, 0, G[from].Count - 1));
}
//増加パスをDFSで探す
long dfs(int v, int t, long f)
{
if (v == t) return f;
used[v] = true;
for (int i = 0; i < G[v].Count; i++)
{
Edge_ e = G[v][i];
if (!used[e.to] && e.cap > 0)
{
long d = dfs(e.to, t, Math.Min(f, e.cap));
if (d > 0)
{
e.cap -= d;
G[e.to][e.rev].cap += d;
return d;
}
}
}
return 0;
}
// sからtへの最大流を求める
public long max_flow(int s, int t)
{
long flow = 0;
while (true)
{
for (int i = 0; i < used.Length; i++) used[i] = false;
long f = dfs(s, t, INF);
if (f == 0) return flow;
flow += f;
}
}
}
class Scanner
{
string[] s; int i;
char[] cs = new char[] { ' ' };
public Scanner() { s = new string[0]; i = 0; }
public string[] scan { get { return ReadLine().Split(); } }
public int[] scanint { get { return Array.ConvertAll(scan, int.Parse); } }
public long[] scanlong { get { return Array.ConvertAll(scan, long.Parse); } }
public double[] scandouble { get { return Array.ConvertAll(scan, double.Parse); } }
public string next
{
get
{
if (i < s.Length) return s[i++];
string st = ReadLine();
while (st == "") st = ReadLine();
s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
i = 0;
return next;
}
}
public int nextint { get { return int.Parse(next); } }
public long nextlong { get { return long.Parse(next); } }
public double nextdouble { get { return double.Parse(next); } }
}