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// Arti1990, II UWr

#include <bits/stdc++.h>

#define forr(i, n)                  for(int i=0; i<n; i++)
#define FOREACH(iter, coll)         for(auto iter = coll.begin(); iter != coll.end(); ++iter)
#define FOREACHR(iter, coll)        for(auto iter = coll.rbegin(); iter != coll.rend(); ++iter)
#define lbound(P,K,FUN)             ({auto SSS=P, PPP = P-1, KKK=(K)+1; while(PPP+1!=KKK) {SSS = (PPP+(KKK-PPP)/2); if(FUN(SSS)) KKK = SSS; else PPP = SSS;} PPP;})
#define testy()                     int _tests; cin>>_tests; FOR(_test, 1, _tests)
#define CLEAR(tab)                  memset(tab, 0, sizeof(tab))
#define CONTAIN(el, coll)           (coll.find(el) != coll.end())
#define FOR(i, a, b)                for(int i=a; i<=b; i++)
#define FORD(i, a, b)               for(int i=a; i>=b; i--)
#define MP                          make_pair
#define PB                          push_back
#define ff                          first
#define ss                          second
#define deb(X)                      X;
#define SIZE(coll) 					((int)coll.size())

#define M 1000000007
#define INF 1000000007LL
#define EPS 1e-9

using namespace std;

int n, l;
vector <int> perm;
string str[27];
int wolne[1000007];

double dokladaj(int nr, double dl, double poz, deque<double> wlozone)
{
    int pocz = poz+EPS;

    while(pocz < l)
    {
        while(!wlozone.empty() && wlozone.front()-EPS < pocz)
            wlozone.pop_front();

        if(str[nr][pocz] == 'X')
        {
            pocz++;
            poz = pocz;
        }
        else if(wolne[pocz] - wlozone.size() <= 1)
        {
            pocz++;
            if(!wlozone.empty() && wlozone.front() < pocz)
                poz = wlozone.front();
            else
                poz = pocz;
        }
        else
            pocz++;

        //cout << "  poz: " << poz << ", pocz: " << pocz << '\n';

        if(pocz - poz > dl - EPS)
            return poz + dl;        
    }
    return l+3;
}

bool testuj(double dl)
{
    sort(perm.begin(), perm.end());
    do
    {
        //cout << "testuje: ";
        //for(auto el : perm)
        //    cout << el << ' ';
        //cout << ", dl: " << dl << '\n';

        deque <double> wlozone;
        double poz = 0;
        bool ok = 1;
        for(auto el : perm)
        {
            auto p = dokladaj(el, dl, poz, wlozone);
            if(p > l+1)
            {
                ok = 0;
                break;
            }
            // cout << "    dolozyl " << el << ", ma koniec na " << p << '\n';
            wlozone.PB(p);
            poz = p-dl;
            while(!wlozone.empty() && wlozone.front() < poz)
                wlozone.pop_front();
        }
        if(ok)
            return true;

    } while (next_permutation(perm.begin(), perm.end()));
    
    return false; //dl <= 2.33333334;
}

int solve()
{
    perm.clear();

    cin >> n >> l;

    forr(i, l)
        wolne[i] = n;
    forr(i, n)
    {
        cin >> str[i];
        perm.PB(i);
        forr(j, l)
            if(str[i][j] == 'X')
            {
                wolne[j]--;
                if(wolne[j] == 0)
                {
                    cout << -1 << '\n';
                    return 0;
                }
            }
    }

    //if(n > 10)
    //    return 0;

    map <long double, pair<int, int>> mapa;
    vector <long double> ulamki;
    FOR(i, 1, n)
        FOR(j, 0, i-1)
            {
                int d = gcd(j, i);
                mapa[(long double)j/i - EPS] = {j/d, i/d};
            }
    for(auto el : mapa)
        ulamki.PB(el.ff);
    //for(auto el : mapa)
    //    cout << el.ss.ff << "/" << el.ss.ss << '\n';

    int pocz = 0, kon = l+1, s;
    while(pocz + 1 < kon)
    {
        s = (pocz + kon)/2;
        if(testuj(s))
            pocz = s;
        else
            kon = s;
    }

    int dl = pocz;
    
    pocz = 0;
    kon = ulamki.size();
    while(pocz + 1 < kon)
    {
        s = (pocz + kon)/2;
        if(testuj(dl + ulamki[s]))
            pocz = s;
        else
            kon = s;
    }

    auto p = mapa[ulamki[pocz]];
    cout << dl*p.ss + p.ff << "/" << p.ss << '\n';

    return 0;
}

int main()
{
    std::ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    //testy()
        solve();

    return 0;
}