/*? begin "base.hpp" */
#ifndef __clang__
#pragma GCC optimize ("O3")
#endif
void solve( /* この関数に問題ごとの処理を書く */
#ifdef GCJ_CASE
 long long case_id
#endif
);

#if defined(EBUG) && !defined(ONLINE_JUDGE)
 #define debug true
#else
 #define NDEBUG 
 #define debug false
#endif
#include<algorithm>
#include<iomanip>
#include<iostream>
#include<limits>
#include<map>
#include<numeric>
#include<queue>
#include<set>
#include<type_traits>
#include<vector>
#ifdef __cpp_lib_execution
  #include<execution>
#endif
#include<cassert>
#include<climits>
#include<cmath>
#include<cstdio>
#include<cstdlib>

using namespace std;
using LL=long long;
using ULL=unsigned long long;
#define int LL 
#define CS const
#define CX constexpr
#define IL inline
#define OP operator
#define RT return
#define TL template
#define TN typename
#define lambda [&]
#define times(n,i) uptil(0,n,i)
#define rtimes(n,i) downto((n)-1,0,i)
#define upto(f,t,i) for(int rabT##i=(t),i=(f);i<=rabT##i;i++)
#define uptil(f,t,i) for(int rabT##i=(t),i=(f);i< rabT##i;i++)
#define downto(f,t,i) for(int rabT##i=(t),i=(f);i>=rabT##i;i--)
#define downtil(f,t,i) for(int rabT##i=(t),i=(f);i> rabT##i;i--)
#define iter(v) begin(v),end(v)
#define citer(v) cbegin(v),cend(v)
#define riter(v) rbegin(v),rend(v)
#define criter(v) crbegin(v),crend(v)
#define IF(a,b,c) ((a)?(b):(c))
#define BINOP_ASGN(t,u,op) t operator op(CS u&o)CS{RT t(*this)op##=o;}
#if debug
 #define _GLIBCXX_DEBUG
 #define _LIBCPP_DEBUG 2
 #define _LIBCPP_DEBUG2 2
 #define ln <<endl
#else
 #define ln <<'\n'
#endif
#define tb <<'\t'
#define sp <<' '
 #define PARABLE 
/*? begin "mod.hpp" */
#ifdef MOD
 #if !defined(FORCE_MOD)&&MOD!=1000000007&&MOD!=1000000009&&MOD!=998244353
  #error unknown mod MOD and FORCE_MOD not defined.
 #endif
#else
 #define MOD 1000000007
#endif
/*? begin "power.hpp" */
using int128=__int128;
TL<TN T>T power(T x,int n){T rt(1);for(;n;n/=2){if(n%2)rt*=x;x*=x;}RT rt;}
int pow_mod(int x,int n,int m){int rt=1;for(;n;n/=2){if(n%2)rt=rt*x%m;x=x*x%m;}RT rt;}
int128 pow_mod_64(int128 x,int n,int m){int128 rt=1;for(;n;n/=2){if(n%2)rt=rt*x%m;x=x*x%m;}RT rt;}
/*? end "power.hpp" */
IL CX int modulo(int a,int m){RT(a%=m,a>=0?a:a+m);}
TL<ULL mod=MOD>class MInt{
  /*! https://ei1333.github.io/luzhiled/snippets/other/mod-int.html */
public:
  int val;
  CX MInt():val(0){}
  explicit CX MInt(int v):val(modulo(v,mod)){}
  MInt&operator+=(CS MInt&m){val+=m.val;if(val>=mod)val-=mod;RT*this;}
  MInt&operator-=(CS MInt&m){val-=m.val;if(val<0)val+=mod;RT*this;}
  MInt&operator*=(CS MInt&m){val=val*m.val%mod;RT*this;}
  MInt&operator/=(CS MInt&m){val=val*m.inv().val%mod;RT*this;}
  BINOP_ASGN(MInt,MInt,+) BINOP_ASGN(MInt,MInt,-) BINOP_ASGN(MInt,MInt,*) BINOP_ASGN(MInt,MInt,/)
  MInt operator-()CS{MInt m;m.val=val?mod-val:0;RT m;}
  bool operator==(CS MInt&m)CS{RT val==m.val;}
  bool operator!=(CS MInt&m)CS{RT val!=m.val;}
  //MInt pow(int n)CS{MInt x(*this),rt(1);while(n){if(n%2)rt*=x;x*=x;n/=2;}RT rt;}
  MInt pow(int n)CS{RT power(*this,n);}
  MInt inv()CS{int a=val,b=mod,x=1,y=0,t;while(b){t=a/b;swap(b,a-=t*b);swap(y,x-=t*y);}RT(MInt)x;}
  friend ostream&operator<<(ostream&o,CS MInt<mod>&m){RT o<<m.val;}
  friend istream&operator>>(istream&i,MInt<mod>&m){int v;i>>v;m=MInt<mod>(v);RT i;}
};
using mint=MInt<>;

constexpr mint operator"" _m(ULL n){RT mint(n);}
constexpr MInt<998244353>operator"" _m998244353(ULL n){RT MInt<998244353>(n);}
constexpr MInt<1000000007>operator"" _m1e9_7(ULL n){RT MInt<1000000007>(n);}
constexpr MInt<1000000009>operator"" _m1e9_9(ULL n){RT MInt<1000000009>(n);}

//#pragma rab:gsub \b(\d+)m\b mint(\1)
/*? end "mod.hpp" */
/*? begin "typedefs.hpp" */
struct unit{};

using int128=__int128;
using LD=long double;
TL<TN T>using vec=vector<T>;
TL<TN T>using vvec=vec<vec<T>>;
TL<TN T>using vvvec=vec<vvec<T>>;
TL<TN T>using vvvvec=vec<vvvec<T>>;

//#pragma rab typedefs.dynamic
using WI = vvec<int>; using VI = vec<int>; using VC = vec<char>; using HII = map<int, int>; using VS = vec<string>; 
/*? end "typedefs.hpp" */
/*? begin "alias.hpp" */
#define PB push_back
#define foldl accumulate
#define scanl partial_sum
/*? end "alias.hpp" */
/*? begin "util.hpp" */
TL<TN T>IL bool amax(T&v,CS T&a){RT v<a&&(v=a,true);}
TL<TN T>IL bool amin(T&v,CS T&a){RT v>a&&(v=a,true);}

#ifndef __cpp_lib_exchange_function
 #define exchange exchange_RAB
 TL<TN T,TN U=T>IL T exchange(T& t, U&& u){T x=move(t);t=forward<U>(u);RT x;}
#endif
#ifndef __cpp_lib_clamp
 #define clamp clamp_RAB
 TL<TN T>IL CX CS T&clamp(CS T&a,CS T&mn,CS T&mx){RT a<mn?mn:a>mx?mx:a;}
#endif

TL<TN T>IL int size_RAB(T t){RT t.size();}
#define size size_RAB

TL<TN V>IL void uniq_after_sort(V&v){v.erase(unique(iter(v)),v.end());}
TL<TN V>IL void sort_and_uniq(V&v){sort(iter(v));v.erase(unique(iter(v)),v.end());}
TL<TN V,TN K>IL auto leftmost_ge(CS V&v,CS K&k){RT v.lower_bound(k);}
TL<TN V,TN K>IL auto leftmost_gt(CS V&v,CS K&k){RT v.upper_bound(k);}

namespace rab{

TL<TN V,TN W>IL void append(V&v,CS W&w){copy(PARABLE citer(w),back_inserter(v));}

TL<TN V>IL auto flatten(CS V&xss,int reserve_size=0)->TN V::value_type{
  decltype(flatten(xss))ret;
  ret.reserve(reserve_size);
  for(CS auto&xs:xss)append(ret,xs);
  ret.shrink_to_fit();
  RT move(ret);
}

TL<TN I>IL bool is_in(I x,I l,I r){RT l<=x&&x<r;}

TL<TN T>IL T fetch(CS T&d,CS vec<T>&v,int i){RT 0<=i&&i<size(v)?v[i]:d;}
TL<TN T>IL T fetch(CS T&d,CS vvec<T>&v,int i,int j){
  RT 0<=i&&i<size(v)&&0<=j&&j<size(v[i])?v[i][j]:d;
}
// TL<TN T,TN U,TN...I>IL T fetch(CS T&d,CS vec<vec<U>>&v,int i,I...j){
// RT 0<=i&&i<size(v)?fetch(d,v[i],j...):d;
// }
TL<TN T>struct Compressed{int size;map<T,int>zip;vec<T>unzip;};
TL<TN T>IL Compressed<T>compressed(vec<T>v){
  sort_and_uniq(v);map<T,int>zip;times(size(v),i)zip[v[i]]=i;RT{size(v),zip,move(v)};
}
TL<TN T>struct CompressedSrc{int size;map<T,int>zip;vec<T>unzip;WI src;};
TL<TN T>IL CompressedSrc<T>compressed_src(CS vec<T>&v){
  auto c=compressed(v);VI src(c.size);times(size(v),i)src[c.zip[v[i]]].PB(i);RT{c.size,c.zip,c.unzip,src};
}

struct identity{TL<TN U>U operator()(U&&v)CS{RT v;}};
}
/*? end "util.hpp" */
/*? begin "debug.hpp" */
TL<class T>
IL istream&operator>>(istream&s,vec<T>&v){for(auto&&p:v)s>>p;RT s;}
TL<class T,class S>
IL ostream&operator<<(ostream&s,CS pair<T,S>&p){RT s<<"("<<p.first<<","<<p.second<<")";}
TL<class T>
IL ostream&operator<<(ostream&,CS vec<T>&);
TL<class T,class S>
IL ostream&operator<<(ostream&,CS map<T,S>&);
#define DEFINE_ITER_OUTPUT(s,x,sep){int i=0;for(CS auto&x##0_elem:x){if(i++)s<<sep;s<<x##0_elem;}RT s;}
TL<class T>
IL ostream&operator<<(ostream&s,CS vec<T>&v)DEFINE_ITER_OUTPUT(s,v,' ')
TL<class T,class S>
IL ostream&operator<<(ostream&s,CS map<T,S>&m)DEFINE_ITER_OUTPUT(s,m,' ')
TL<class T>
IL ostream&operator<<(ostream&s,CS vec<vec<T>>&w)DEFINE_ITER_OUTPUT(s,w,'\n')
TL<class T,class S>
IL ostream&operator<<(ostream&s,CS vec<map<T,S>>&v)DEFINE_ITER_OUTPUT(s,v,'\n')
/*? end "debug.hpp" */

signed main(){
 {if(debug)cerr<<"MOD: "<<(MOD)ln;}
 if(!debug)cin.tie(0),cerr.tie(0),ios::sync_with_stdio(0);
 cout<<fixed<<setprecision(20);
 cerr<<fixed<<setprecision(20);

 #ifdef GCJ_CASE
  int T;cin>>T;
  times(T,t){cout<<"Case #"<<t+1<<": ";solve(t);}
 #else
  solve();
 #endif

 return 0;
}
/*? end "base.hpp" */
/*? begin "graph.hpp" */
/*? begin "uf.hpp" */
TL<class T=int,class Adder=plus<T>,class Inverser=negate<T>>
class UnionFind{
/*!
http://noshi91.hatenablog.com/entry/2018/05/30/191943
https://en.wikipedia.org/wiki/Disjoint-set_data_structure
https://qiita.com/drken/items/cce6fc5c579051e64fab
*/
int n,*parents,*sizes;T*pot_diffs;bool to_delete;Adder adder;
Inverser inverser;
public:
explicit UnionFind(int n,bool to_delete=false):
n(n),parents(new int[n]),sizes(new int[n]),pot_diffs(new T[n]),to_delete(to_delete)
{clear();
}
void clear(){
times(n,i)parents[i]=i;/*roots*/
fill(sizes,sizes+n,1);fill(pot_diffs,pot_diffs+n,0);
}
~UnionFind(){if(to_delete){delete[]parents;delete[]sizes;delete[]pot_diffs;}}
int size(){RT n;}
int root(int i){int p=parents[i];
if(p==i)RT i;/*`i`is a root*/
int r=root(p);/*and pot_diffs[p]:=diff from root*/
pot_diffs[i]+=pot_diffs[p];parents[i]=r;RT r;
}
bool is_same(int i,int j){RT root(i)==root(j);}
bool is_all_same(){int r=root(0);uptil(1,n,i)if(root(i)!=r)RT 0;RT 1;}
bool merge(int i,int j,T pdiff=0){i=root(i);j=root(j);
if(i==j)RT false;/*already merged*/
if(sizes[i]>sizes[j]){swap(i,j);pdiff=inverser(pdiff);
}
/*now sizes[i]<=sizes[j]*/
parents[i]=j;sizes[j]+=sizes[i];pot_diffs[i]=pdiff;RT true;
}
T diff(int i,int j){
root(i);/*pot_diffs[i]:=diff from root*/
root(j);/*pot_diffs[j]:=diff from root*/
RT adder(pot_diffs[i],inverser(pot_diffs[j]));}};using unionfind=UnionFind<>;
/*? end "uf.hpp" */
TL<class EdgeVal>
struct Edge{int from;int to;EdgeVal weight;
Edge(int from,int to,EdgeVal weight):from(from),to(to),weight(weight){}
IL bool OP==(CS Edge&e)CS{RT weight==e.weight&&from==e.from&&to==e.to;}
IL bool OP<(CS Edge&e)CS{RT weight<e.weight||(weight==e.weight&&(from<e.from||(from==e.from&&to<e.to)));}
IL bool OP<=(CS Edge&e)CS{RT this==e||this<e;}
IL bool OP>(CS Edge&e)CS{RT e<this;}
IL bool OP>=(CS Edge&e)CS{RT e<=this;}};
TL<class VtxVal,class EdgeVal>
class Graph{
using VoidWithoutEdgeVal=TN enable_if<is_default_constructible<EdgeVal>::value,void>::type;
protected:
int nv_,nde_;unionfind uf;
public:
vec<VtxVal>vs;vvec<Edge<EdgeVal>>edges;
Graph(int nv):nv_(nv),nde_(0),uf(nv),vs(nv),edges(nv){}
IL int nv()CS{RT nv_;}
IL int nde()CS{RT nde_;}
IL int nue()CS{RT nde_/2;}
IL void add_dedge(int i,int j,CS EdgeVal&val){
if(!rab::is_in(i,0LL,nv_)||!rab::is_in(j,0LL,nv_)){
cerr<<"invalid index:("<<i<<","<<j<<")for Graph(nv="<<nv_<<")" ln;
exit(1);
}
edges[i].emplace_back(i,j,val);++nde_;
}
IL VoidWithoutEdgeVal add_dedge(int i,int j){add_dedge(i,j,EdgeVal());
}
IL void add_uedge(int i,int j,CS EdgeVal&val){add_dedge(i,j,val);
add_dedge(j,i,val);
}
IL VoidWithoutEdgeVal add_uedge(int i,int j){add_uedge(i,j,EdgeVal());
}
IL bool is_connected(){uf.clear();
for(CS auto&es:edges)for(CS auto&e:es)uf.merge(e.from,e.to);RT uf.is_all_same();
}};
/*? end "graph.hpp" */
/*? begin "flow.hpp" */
/*!Arihon,https://tubo28.me/algorithm/dinic/*/
TL<TN F,TN V,TN E,TN CapFn=rab::identity>
/*F:通常int,CapFn:E->F*/
struct dinic{int n,s,t;VI level,prog,que;
WI edges;/*both direction*/
vvec<F>cap,flow;
dinic(CS Graph<V,E>&g,CS CapFn&capfn=rab::identity()):
n(g.nv()),s(-1),t(-1),level(n),prog(n),que(n+1),edges(n),cap(n,VI(n)),flow(n,VI(n))
{times(n,i){for(auto&e:g.edges[i]){F c=capfn(e.weight);
cap[i][e.to]+=c;cap[e.to][i]+=c;flow[e.to][i]+=c;edges[i].PB(e.to);
edges[e.to].PB(i);
}}}
F exec(int s,int t){this->s=s;this->t=t;F mf=0;
while(update_level(),level[t]>0){fill(iter(prog),0);mf+=find_paths(s,numeric_limits<F>::max()/8/*inf*/);
}
RT mf;
}
void update_level(){int ql=0,qr=0;fill(iter(level),-1);level[s]=0;
que[qr++]=s;while(ql!=qr/*queue is not empty*/){int v=que[ql++];
if(v==t)break;
for(int d:edges[v]){if(level[d]<0&&cap[v][d]!=flow[v][d]){level[d]=level[v]+1;
que[qr++]=d;
}}}}
F find_paths(int v,int limit){if(v==t)RT limit;F diff=0;
for(;prog[v]<size(edges[v]);++prog[v]){int d=edges[v][prog[v]];
if(cap[v][d]!=flow[v][d]&&level[v]<level[d]){F df=find_paths(d,min(limit,cap[v][d]-flow[v][d]));
flow[v][d]+=df;flow[d][v]-=df;diff+=df;limit-=df;if(limit==0)break;
}}
RT diff;}};
/*!Arihon,https://tubo28.me/algorithm/ford-fulkerson/*/
TL<TN V,TN E,TN CapFn=rab::identity>
struct ford_fulkerson{int n,s,t;VC visited;
WI edges;/*both direction*/
private:
HII cap_,flow_;
public:
ford_fulkerson(CS Graph<V,E>&g,CS CapFn&capfn=rab::identity()):
n(g.nv()),s(-1),t(-1),visited(n),edges(n)
{times(n,i){for(auto&e:g.edges[i]){int c=capfn(e.weight);
cap_[i<<32|e.to]+=c;cap_[e.to<<32|i]+=c;flow_[e.to<<32|i]+=c;edges[i].PB(e.to);
edges[e.to].PB(i);
}}}
int cap(int a,int b){RT cap_[a<<32|b];}
int flow(int a,int b){RT flow_[a<<32|b];}
int exec(int s,int t){this->s=s;this->t=t;int mf=0;
while(1){fill(iter(visited),false);int df=find_path(s,LLONG_MAX/8/*inf*/);if(df==0)RT mf;
mf+=df;
}}
int find_path(int v,int limit){visited[v]=true;if(v==t)RT limit;
for(int d:edges[v]){if(!visited[d]){int c=cap_[v<<32|d],&f=flow_[v<<32|d];
if(c-f>0){int df=find_path(d,min(limit,c-f));if(df>0){f+=df;flow_[d<<32|v]-=df;RT df;
}}}}
RT 0;}};
/*? end "flow.hpp" */
constexpr int INF = 1ll << 50;

void solve() {
// HWH("S")
/* <foxy.memo-area> */
int H;int W;cin>>H;cin>>W;VS S(H);times(H,Ri_0){cin>>S[Ri_0];}
/* </foxy.memo-area> */

  Graph<unit, int> g(H * W * 2 + 2);

  int s = H * W * 2, t = H * W * 2 + 1;

  #define inp(i, j) (((i) * W + (j)) * 2)
  #define outp(i, j) (((i) * W + (j)) * 2 + 1)

  times(H, i) times(W, j) {
    g.add_dedge(inp(i, j), outp(i, j), 1);

    if(S[i][j] == 'X') {
      g.add_dedge(s, outp(i, j), INF);

      if(i == 0 || i == H - 1 || j == 0 || j == W - 1) {
        cout << -1 ln;
        return;
      }
    }
  }

  times(H, i) {
    g.add_dedge(outp(i, 0), t, INF);
    g.add_dedge(outp(i, W-1), t, INF);

    times(W-1, j) {
      g.add_dedge(outp(i, j), inp(i, j + 1), INF);
      g.add_dedge(outp(i, j + 1), inp(i, j), INF);
    }
  }
  times(W, j) {
    g.add_dedge(outp(0, j), t, INF);
    g.add_dedge(outp(H-1, j), t, INF);

    times(H-1, i) {
      g.add_dedge(outp(i, j), inp(i + 1, j), INF);
      g.add_dedge(outp(i + 1, j), inp(i, j), INF);
    }
  }

  ford_fulkerson<unit, int> d(g);
  cout << d.exec(s, t) ln;
}


Measured memory_bytes: 18560