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// {{{ file: opi.cpp | time: 18:41 13.03.2025
#define _USE_MATH_DEFINES
#include <bits/stdc++.h>

#define each(...)    for (auto& __VA_ARGS__)
#define rep(i, b, e) for (int i = (b); i <= (e); i++)
#define rev(i, b, e) for (int i = (e); i >= (b); i--)
#define mp make_pair
#define mt make_tuple
#define x first
#define y second
#define pb push_back
#define all(x) (x).begin(), (x).end()
#define endl '\n'
#define tC template<class
#define $(x) #x<<'='<<(x)<<' '

using namespace std;
using ll = long long;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using vb = vector<bool>;
using vi = vector<int>;
using vl = vector<ll>;
using vs = vector<string>;
using vpi = vector<pii>;
using vpl = vector<pll>;
using vvi = vector<vi>;
using vvl = vector<vl>;
tC T, class C=greater<T>> using min_priority_queue = priority_queue<T,vector<T>,C>;
tC T> int sz(const T& a) { return (int)a.size(); }
tC T> bool amin(T& a, T b) { return b < a ? a = b, 1 : 0; }
tC T> bool amax(T& a, T b) { return b > a ? a = b, 1 : 0; }
const int oo = 1e9+1;
const ll OO = ll(1e18)+1;

auto now() { return chrono::high_resolution_clock::now().time_since_epoch().count(); }
mt19937 rnd(4488);
tC T> T rand(T lo, T hi) { return uniform_int_distribution<T>{lo,hi}(rnd); }

struct Debug {
#ifdef SPONGE
  tC T>Debug operator<<(const T& x) { cerr<<"\033[1;33m"<<x<<"\033[0m"; return *this; }
#else
  tC T>Debug operator<<(const T&) { return *this; }
#endif
} dbg;

namespace { void solve(); }
// }}}

int main()
{
  //freopen("input.txt","r",stdin);
  //freopen("output.txt","w",stdout);
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.precision(15);
  cout.setf(ios::fixed,ios::floatfield);
  cerr.precision(15);
  cerr.setf(ios::fixed,ios::floatfield);
  //rnd.seed(now());
  int t=1;
  //cin>>t;
  rep(i,1,t){
    //cerr<<"Case #"<<i<<": ";
    solve();
  }
}

namespace {

using arr_t = array<int,18>;

const int N=18e5+5;
int n,m,l,a[18][N],suma[N],last[19][N],wolne[18][N],wsk[18][N];
arr_t dp[1<<18];
string wej[18];

void info_pamiec()
{
  int uzyta=0;
  uzyta+=sizeof(a);
  uzyta+=sizeof(suma);
  uzyta+=sizeof(last);
  uzyta+=sizeof(wolne);
  uzyta+=sizeof(wsk);
  uzyta+=sizeof(dp);
  uzyta+=sizeof(wsk);
  double megabajty=uzyta/1024.0/1024.0;
  cerr<<$(megabajty)<<endl;
}

void skaluj_wej(int x)
{
  m=l*x;
  rep(i,0,n-1) rep(j,0,l-1) rep(k,0,x-1){
    if(wej[i][j]=='X') a[i][j*x+k+1]=1;
    else a[i][j*x+k+1]=0;
  }
}

bool dasie(int t)
{
  rep(j,1,m) suma[j]=0;
  rep(i,0,n-1) rep(j,1,m) suma[j]+=a[i][j];

  rep(i,1,m) rep(x,0,18){
    if(suma[i]>=x) last[x][i]=i;
    else last[x][i]=last[x][i-1];
  }

  if(t==0 && last[n][m]==0) return true;
  if(last[n][m]>0) return false;

  rep(i,0,n-1) wolne[i][m+1]=0;
  rep(i,0,n-1) rev(j,1,m){
    if(a[i][j]==1 || suma[j]==n-1) wolne[i][j]=0;
    else wolne[i][j]=wolne[i][j+1]+1;
  }

  rep(i,0,n-1) wsk[i][m+1]=0;
  rep(i,0,n-1) rev(j,0,m){
    if(wolne[i][j]>=t) wsk[i][j]=j;
    else wsk[i][j]=wsk[i][j+1];
  }

  rep(maska,0,(1<<n)-1) rep(i,0,n-1) dp[maska][i]=oo;

  rep(maska,0,(1<<n)-2){
    /*
    dbg<<$(maska)<<endl;
    rep(i,0,n-1) dbg<<$(i)<<':'<<$(dp[maska][i])<<' ';
    dbg<<endl;
    */

    if(maska>0 && dp[maska][0]==oo) continue;

    int k=0;
    rep(i,0,n-1) if(maska&(1<<i)) k++;

    int start;
    if(maska>0) start=dp[maska][0]; else start=0;

    int nowy_start=start;
    rep(i,0,k-1){
      int koniec=dp[maska][i]+t-1;
      if(koniec>=start) amax(nowy_start,last[n-1-k+i][koniec]+1);
    }

    rep(i,0,n-1) if((maska&(1<<i)) == 0 && wsk[i][nowy_start]!=0){
      arr_t stan=dp[maska];
      stan[k]=wsk[i][nowy_start];
      if(stan<dp[maska|(1<<i)]) dp[maska|(1<<i)]=stan;
    }
  }

  return (dp[(1<<n)-1][0] < oo);
}

//int nww=16*9*5*7*11*13*17;

tC F>int wyszukaj(int lew,int pra,F&& fun)
{
  int odp=-1;
  while(lew<=pra){
    int sro=(lew+pra)/2;
    if(fun(sro)){
      odp=sro;
      lew=sro+1;
    }
    else{
      pra=sro-1;
    }
  }
  return odp;
}

bool porownaj(pii a1,pii b1)
{
  return a1.y * b1.x < b1.y * a1.x;
}

void solve()
{
  //info_pamiec();

  cin>>n>>l;
  rep(i,0,n-1) cin>>wej[i];

  skaluj_wej(1);
  int calk_odp=wyszukaj(0,l,dasie);

  if(calk_odp==-1){
    cout<<-1<<endl;
  }
  else{
    vpi ulamki;
    ulamki.pb(mp(1,0));
    rep(mian,1,18) rep(licz,1,mian-1){
      if(gcd(mian,licz)==1) ulamki.pb(mp(mian,licz));
    }
    sort(all(ulamki),porownaj);

    int indeks=wyszukaj(0,sz(ulamki)-1,[&](int i){
      auto [mian,licz]=ulamki[i];
      skaluj_wej(mian);
      return dasie(calk_odp*mian+licz);
    });

    int licz_odp=calk_odp*ulamki[indeks].x+ulamki[indeks].y;
    int mian_odp=ulamki[indeks].x;
    cout<<licz_odp<<'/'<<mian_odp<<endl;
  }
}

}