#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <cmath>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <iostream>
#include <random>
#include <map>
#include <iomanip>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
#include <cassert>
#include <fstream>
#include <unordered_map>
#include <cstdlib>
#include <complex>
#include <cctype>
#include <bitset>
using namespace std;

using ll = long long;
using vll = vector<ll>;
using pll = pair<ll, ll>;
using qll = queue<ll>;
using vb = vector<bool>;
using mll = map<ll, ll>;
using sll = stack<ll>;
#define REP(i,n) for(ll i(0);(i)<(n);(i)++)
#define rep(i,n) for(ll i(0);(i)<(n);(i)++)
#define ALL(a) a.begin(), a.end()
#define enld endl //* missspell check
const ll INF = 1LL << 60;
ll H, W;
vll dx{1, 0, -1, 0}, dy{0, 1, 0, -1};

struct edgeForFlow{ll to, cap, rev; }; //! cap: may change; rev: pointer in G[to]

void addEdgeForFlow(vector<vector<edgeForFlow>> &G, ll from, ll to, ll cap){
    G[from].push_back((edgeForFlow){ to, cap,  (ll) G[to].size()});
    G[to].push_back((edgeForFlow) { from, 0, (ll) G[from].size()-1});
}

ll dfsFordFulkson(vector<vector<edgeForFlow>> &G, vb &checked, ll v, ll t, ll f){
    //* v: current vertex, t: sink, f: DELTA of this path. (No need for source)
    if(v == t) return f;
    checked[v] = true;
    REP(i, G[v].size()){
        edgeForFlow &e = G[v][i];
        if(!checked[e.to] && e.cap > 0){
            ll d = dfsFordFulkson(G, checked, e.to, t, min(f, e.cap));
            if(d > 0){
                e.cap -= d;
                G[e.to][e.rev].cap += d;
                return d;
            }
        }
    }
    return 0; //* if no valid outgoing edges
}

ll maxFlowFordFulkson(vector<vector<edgeForFlow>> &G, ll s, ll t){
    vb checked(G.size());
    ll flow = 0;
    for(;;){
        fill(ALL(checked), false);
        ll f = dfsFordFulkson(G, checked, s, t, INF);
        if(f == 0) 
            return flow;
        flow += f;
    }
}

void bfsDinic(vector<vector<edgeForFlow>> &G, vll &level, ll s){
    fill(ALL(level), -1);
    queue<ll> que;
    level[s] = 0;
    que.push(s);
    while(!que.empty()){
        ll v = que.front(); que.pop();
        for(ll i=0; i< G[v].size(); i++){
            edgeForFlow &e = G[v][i];
            if(e.cap > 0 && level[e.to] < 0){
                level[e.to] = level[v] + 1;
                que.push(e.to);
            }
        }
    }
}

ll dfsDinic(vector<vector<edgeForFlow>> &G, vll &level, vll &iter, ll v, ll t, ll f){
    //? iter: record where have been searched?
    //* v: current vertex, t: sink, f: DELTA of this path. (No need for source)
    if(v == t) return f;
    for( ll &i = iter[v]; i < G[v].size(); i++){
        edgeForFlow &e = G[v][i];
        if( e.cap > 0 && level[v] < level[e.to]){
            ll d = dfsDinic(G, level, iter, e.to, t, min(f, e.cap));
            if(d > 0){
                e.cap -= d;
                G[e.to][e.rev].cap += d;
                return d;
            }
        }
    }
    return 0; //* if no valid outgoing edges
}

ll maxFlowDinic(vector<vector<edgeForFlow>> &G, ll s, ll t){
    vll level(G.size()), iter(G.size());
    ll flow = 0;
    for(;;){
        bfsDinic(G, level, s);
        if(level[t] < 0)//* sink t is not reachable from s on current residual graph
            return flow;
        fill(ALL(iter), 0);
        ll f;
        while((f = dfsDinic(G, level, iter, s, t, INF)) > 0){
            flow += f;
            // cout<< "flow: "<<flow<<endl;
        }
    }
}

ll xyToIdx(ll x, ll y){
    return (x*W + y);
}

void idxToXY(ll idx, ll &x, ll &y){
    x = idx / W;
    y = idx % W;
}

void solve(long long H, long long W, std::vector<std::string> S){
    ll N = H*W;
    //* source: 2*N, sink: 2*N+1
    vector<vector<edgeForFlow>> G(2*N+2);
    REP(x, H){
        REP(y, W){
            ll u = xyToIdx(x, y), xto, yto;
            REP(k, 4){
                xto = x + dx[k]; yto = y + dy[k];
                if(0 <= xto && xto < H && 0 <= yto && yto < W){
                    ll v = xyToIdx(xto, yto);
                    addEdgeForFlow(G, u+N, v, 1);
                }
            }
            if(S[x][y]=='X'){
                addEdgeForFlow(G, 2*N, u, H*W);
                addEdgeForFlow(G, u, u+N, H*W);
            }
            else{
                addEdgeForFlow(G, u, u+N, 1);
            }
            if( x == 0 || x == H-1 || y == 0 || y == W-1){
                addEdgeForFlow(G, u+N, 2*N+1, H*W);
            }
        }
    }

    ll flow = 0;
    flow = maxFlowDinic(G, 2*N, 2*N+1);
    if(flow >= H*W)
        cout<<-1<<endl;
    else
        cout << flow << endl;
}

int main(){
    scanf("%lld",&H);
    scanf("%lld",&W);
    std::vector<std::string> S(H);
    for(int i = 0 ; i < H ; i++){
        std::cin >> S[i];
    }
    solve(H, W, std::move(S));
    return 0;
}


Measured memory_bytes: 5436