#line 1 "base.hpp"//5
#ifndef __clang__
#pragma GCC optimize ("O3")
#endif
void solve(
#ifdef GCJ_CASE
long long case_id
#endif
);
#if defined(EBUG) && !defined(ONLINE_JUDGE)
#define debug true
#define _GLIBCXX_DEBUG
// #define _LIBCPP_DEBUG 0
#define _LIBCPP_DEBUG2 0
#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>
#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 rep(f,t,i,c,u)for(int Rb_=(t),i=(f);i c Rb_;u(i))
#define upto(f,t,i)rep(f,t,i,<=,++)
#define uptil(f,t,i)rep(f,t,i,<,++)
#define downto(f,t,i)rep(f,t,i,>=,--)
#define downtil(f,t,i)rep(f,t,i,>,--)
#define times(n,i)uptil(0,n,i)
#define rtimes(n,i)downto((n)-1,0,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))
#if debug
#define ln <<endl
#else
#define ln <<'\n'
#endif
#define tb <<'\t'
#define sp <<' '
#line 1 "base_util.hpp"//5b
#define BINOPa(t,u,op)t OP op(CS u&o)CS{RT t(*this)op##=o;}
#define CMPOP(t,op,f1,f2,x)bool OP op(CS t&x)CS{RT f1 op f2;}
#define CMPOPS(t,f1,f2,x)CMPOP(t,==,f1,f2,x)CMPOP(t,!=,f1,f2,x)\
CMPOP(t,<,f1,f2,x)CMPOP(t,<=,f1,f2,x)CMPOP(t,>,f1,f2,x)CMPOP(t,>=,f1,f2,x)
#line 1 "mod.hpp"//5b
#ifndef MOD
#define MOD 1000000007
#endif
#if !defined(FORCE_MOD)&&MOD!=1000000007&&MOD!=1000000009&&MOD!=998244353
#error mod
#endif
#line 1 "power.hpp"//5bm
TL<TN T>IL T power(T x,int n){T r(1);for(;n;n/=2){if(n%2)r*=x;x*=x;}RT r;}IL int pow_mod(int x,int n,int m){int r=1;
for(;n;n/=2){if(n%2)r=r*x%m;x=x*x%m;}RT r;}
#line 2001 "mod.hpp"//5b
IL CX int modulo(int a,int b){a%=b;RT a&&(a>0)!=(b>0)?a+b:a;}IL CX int divide(int a,int b){RT(a-modulo(a,b))/b;}
TL<int d=MOD>struct MInt{
/*!https://ei1333.github.io/luzhiled/snippets/other/mod-int.html*/
int v;CX MInt():v(0){}explicit CX MInt(int i):v(modulo(i,d)){}MInt&OP+=(CS MInt&m){v+=m.v;if(v>=d)v-=d;RT*this;}
MInt&OP-=(CS MInt&m){v-=m.v;if(v<0)v+=d;RT*this;}MInt&OP*=(CS MInt&m){v=v*m.v%d;RT*this;}MInt&OP/=(CS MInt&m){
RT*this*=m.inv();}BINOPa(MInt,MInt,+)BINOPa(MInt,MInt,-)BINOPa(MInt,MInt,*)BINOPa(MInt,MInt,/)MInt OP-()CS{
RT MInt()-=*this;}CMPOP(MInt,==,v,m.v,m)CMPOP(MInt,!=,v,m.v,m)MInt pow(int n)CS{RT power(*this,n);}MInt inv()CS{
int a=v,b=d,x=1,y=0;while(b){swap(y,x-=a/b*y);swap(b,a%=b);}RT(MInt)x;}
friend ostream&OP<<(ostream&o,CS MInt&m){RT o<<m.v;}friend istream&OP>>(istream&i,MInt&m){i>>m.v;m.v%=d;RT i;}};
using mint=MInt<>;CX mint OP"" _m(ULL n){RT mint(n);}CX MInt<998244353>OP"" _m998244353(ULL n){RT MInt<998244353>(n);}
CX MInt<1000000007>OP"" _m1e9_7(ULL n){RT MInt<1000000007>(n);}
CX MInt<1000000009>OP"" _m1e9_9(ULL n){RT MInt<1000000009>(n);}
#line 1 "typedefs.hpp"//5b
using unit = tuple<>;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>>;
using VI=vec<int>;using VS=vec<string>;
#line 1 "alias.hpp"//5b
#define EB emplace_back
#define PB push_back
#define foldl accumulate
#define scanl partial_sum
#line 1 "util.hpp"//5b
TL<TN T>IL bool amax(T&v,CS T&a){RT v<a&&(v=a,1);}TL<TN T>IL bool amin(T&v,CS T&a){RT v>a&&(v=a,1);}
TL<TN T>IL int sizeRAB(T t){RT t.size();}
#define size sizeRAB
#define clamp clampRAB
TL<TN T>IL CX CS T&clamp(CS T&a,CS T&l,CS T&r){RT a<l?l:r<a?r:a;}TL<TN V>IL void uniq2(V&v){
v.erase(unique(iter(v)),v.end());}TL<TN V>IL void uniq(V&v){sort(iter(v));uniq2(v);}
#define leftmost_ge lower_bound
#define leftmost_gt upper_bound
namespace rab{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;}}
#line 1 "debug.hpp"//5b
TL<TN T>IL istream&OP>>(istream&s,vec<T>&v){for(auto&&p:v)s>>p;RT s;}TL<TN T,TN S>
IL ostream&OP<<(ostream&s,CS pair<T,S>&p){RT s<<"("<<p.first<<","<<p.second<<")";}
#define Rdebug1(sep, ...)IL ostream& OP<<(ostream&s,CS __VA_ARGS__&v){\
int i=0;for(CS auto&e:v){i++&&s<<sep;s<<e;}RT s;}
TL<TN T>Rdebug1(' ',vec<T>)TL<TN T,TN S>Rdebug1(' ',map<T,S>)TL<TN T>Rdebug1('\n',vvec<T>)
TL<TN T,TN S>Rdebug1('\n',vec<map<T,S>>)
#line 6001 "base.hpp"//5
signed main(){if(debug)cerr<<"MOD: "<<MOD ln;else cin.tie(0),cerr.tie(0),ios::sync_with_stdio(0);
auto p=setprecision(20);cout<<fixed<<p;cerr<<fixed<<p;
#ifdef GCJ_CASE
int T;cin>>T;times(T,t){cout<<"Case #"<<t+1<<": ";solve(t);}
#else
solve();
#endif
RT 0;}
#line 1 "graph.hpp"//5
TL<TN E>struct Edge{int from;int to;E weight;Edge(int from,int to,E weight):from(from),to(to),weight(weight){}
CMPOPS(Edge,make_tuple(weight,from,to),make_tuple(e.weight,e.from,e.to),e)};TL<TN E>class Graph{protected:int nv_,nde_;
public:vvec<Edge<E>>edges;Graph(int nv):nv_(nv),nde_(0),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 E&val){if(debug&&(i<0||nv_<=i||j<0||nv_<=j)){
cerr<<"invalid index:("<<i<<","<<j<<")for Graph(nv="<<nv_<<")" ln;exit(1);}edges[i].emplace_back(i,j,val);++nde_;}
IL void add_uedge(int i,int j,CS E&val){add_dedge(i,j,val);add_dedge(j,i,val);}};
#line 1 "util_identity.hpp"//5f
struct identity{TL<TN U>U OP()(U&&v)CS{RT v;}};
#line 3001 "flow.hpp"//5
/*!Arihon,https://tubo28.me/algorithm/dinic/,http://topcoder.g.hatena.ne.jp/Mi_Sawa/20140311*/
TL<TN F,TN E,TN CapFn=identity>class dinic{public:struct FlowEdge{CS int from,to,nxt;CS F cap;F flow;
FlowEdge(int from,int to,int nxt,F cap,F flow):from(from),to(to),nxt(nxt),cap(cap),flow(flow){}};int n,s,t;
VI level,prog,que,heads;vec<FlowEdge>edges;dinic(CS Graph<E>&g,CS CapFn&capfn=identity()):
n(g.nv()),s(-1),t(-1),level(n),prog(n),que(n),heads(n){fill(iter(heads),-1);edges.reserve(g.nde()*2);int edges_i=0;
times(n,i){for(auto&e:g.edges[i]){F c=capfn(e.weight);edges.EB(i,e.to,heads[i],c,(F)0);heads[i]=edges_i;++edges_i;
edges.EB(e.to,i,heads[e.to],c,c);heads[e.to]=edges_i;++edges_i;}}}F exec(int s,int t){this->s=s;this->t=t;
F mf=0,inf=numeric_limits<F>::max()/8;while(update_level(),level[s]){copy(iter(heads),begin(prog));
mf+=find_paths(s,inf);}RT mf;}private:void update_level(){int ql=0,qr=0;fill(iter(level),0);level[t]=n;que[qr++]=t;
while(ql!=qr){int v=que[ql++];if(v==s)RT;for(int i=heads[v];~i;i=edges[i].nxt){CS auto&e=edges[i];
if(!level[e.to]&&e.flow!=0){level[e.to]=level[v]-1;que[qr++]=e.to;}}}}F find_paths(int v,int limit){if(v==t)RT limit;
F diff=0;for(int i=prog[v];~i;i=prog[v]=edges[i].nxt){auto&e=edges[i];if(level[v]<level[e.to]&&e.cap!=e.flow){
F df=find_paths(e.to,min(limit,e.cap-e.flow));e.flow+=df;edges[i^1].flow-=df;diff+=df;limit-=df;if(limit==0)break;}}
RT diff;}};
#line 3001 "5.cpp"//
constexpr int INF = 401;

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<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);
    }
  }

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


Measured memory_bytes: 6656