#include <algorithm>
#include <bits/stdc++.h>
#include <iomanip>
using namespace std;
#define fwd(i, a, n) for (int i = (a); i < (n); i++)
#define rep(i, n) fwd(i, 0, n)
#define all(X) X.begin(), X.end()
#define sz(X) int(size(X))
#define pb push_back
#define eb emplace_back
#define st first
#define nd second
using pii = pair<int, int>; using vi = vector<int>;
using ll = long long; using ld = long double;
#ifdef LOC
auto SS = signal(6, [](int) { *(int *)0 = 0; });
#define DTP(x, y) auto operator << (auto &o, auto a) -> decltype(y, o) { o << "("; x; return o << ")"; }
DTP(o << a.st << ", " << a.nd, a.nd);
DTP(for (auto i : a) o << i << ", ", all(a));
void dump(auto... x) { (( cerr << x << ", " ), ...) <<
'\n'; }
#define deb(x...) cerr << setw(4) << __LINE__ << ":[" #x "]: ", dump(x)
#else
#define deb(...) 0
#endif
 
struct SegTree{
    vector<int> v;
    vector<int> pos;
    vector<int> mod;
    int N = 2;

    void pull(int p){
        int a = v[2*p] + mod[2*p];
        int b = v[2*p + 1] + mod[2*p + 1];
        if(a < b){
            v[p] = a;
            pos[p] = pos[2*p];
        }else{
            v[p] = b;
            pos[p] = pos[2*p + 1];
        }
    }

    void push(int p){
        v[p] += mod[p];
        mod[2*p] += mod[p];
        mod[2*p + 1] += mod[p];
        mod[p] = 0;
    }

    void rangeAdd(int a, int b, int l, int r, int p, int x){
        if(b < l || r < a)return;
        if(a <= l && r <= b){
            mod[p] += x;
        }else{
            int m = (l + r) / 2;
            push(p);
            rangeAdd(a, b, l, m, 2*p, x);
            rangeAdd(a, b, m+1, r, 2*p + 1, x);
            pull(p);
        }
    }

    pii rangeGet(int a, int b, int l, int r, int p){
        if(b < l || r < a)return {1e9, -1};
        if(a <= l && r <= b){
            return {v[p] + mod[p], pos[p]};
        }else{
            int m = (l + r) / 2;
            auto [x1, y1] = rangeGet(a, b, l, m, 2 * p);
            auto [x2, y2] = rangeGet(a, b, m+1, r, 2*p + 1);
            if(x1 < x2){
                return {x1 + mod[p], y1};
            }else{
                return {x2 + mod[p], y2};
            }
        }
    }

    SegTree(int n, vector<int>& free){
        int d = n / sz(free);
        while(N < n) N *= 2;
        v.resize(2*N, 1e9);
        pos.resize(2*N);
        rep(i, N)pos[N + i] = i;
        rep(i, n){
            v[N + i] = free[i/d];
        }
        mod.resize(2*N, 0);
        for(int j = N-1; j >= 1; j--)pull(j);
    }

    void add(int a, int b, int x){
        rangeAdd(a, b, 0, N-1, 1, x);
    }

    pii get(int a, int b){
        return rangeGet(a, b, 0, N-1, 1);
    }
};

void solve(){
    int n, L; cin >> n >> L;
    vector<string> plan(n);
    rep(i, n)cin >> plan[i];
    vector<int> free(L, 0);
    rep(i, n){
        rep(j, L){
            if(plan[i][j] == '.')free[j]++;
        }
    }
    rep(j, L){
        if(free[j] == 0){
            cout << "-1\n";
            return;
        }
    }
    // if(n > 11){
    //     cout << "BIG\n";
    //     return;
    // }
    deb(free);
    vector<pii> frac;
    auto cmp = [](pii a, pii b){
        return 1ll * a.st * b.nd < 1ll * a.nd * b.st;
    };
    for(int b = 1; b <= n; b++){
        for(int a = 0; 2*a <= b * L; a++){
            if(__gcd(a, b) == 1){
                frac.push_back({a, b});
            }
        }
    }
    sort(all(frac), cmp);
    deb(frac);
    auto check = [&](pii f){
        int a = f.st; int b = f.nd;
        deb(a, b);
        if(a == 0)return true;
        SegTree tree(L * b, free);
        vector<vector<pii>> segments(n);
        rep(i, n){
            int goodCnt = 0;
            for(int j = 0; j < a; j++){
                if(plan[i][j/b] == '.' && free[j/b] > 1)goodCnt++;
            }
            for(int j = 0; j + a < L * b; j++){
                if(goodCnt == a){
                    if(sz(segments[i]) == 0 || segments[i].back().nd + 1 < j){
                        segments[i].push_back({j, j});
                    }else{
                        segments[i].back().nd++;
                    }
                }
                if(plan[i][j/b] == '.' && free[j/b] > 1)goodCnt--;
                if(plan[i][(j+a)/b] == '.' && free[(j+a)/b] > 1)goodCnt++;
            }
            if(goodCnt == a){
                if(sz(segments[i]) == 0 || segments[i].back().nd + 1 < L*b - a){
                    segments[i].push_back({L*b - a, L * b - a});
                }else{
                    segments[i].back().nd++;
                }
            }
            if(sz(segments[i]) == 0)return false;
        }
        deb(segments);
        vector<int> p(n);
        rep(i, n)p[i] = i;
        auto nextPos = [&](int pos, int i){
            auto [x, y] = tree.get(pos, pos + a -1);
            y++;
            if(x > 1)y = pos;
            auto it = lower_bound(all(segments[i]), pii{y+1, 0});
            int res = -1;
            if(it != segments[i].end()){
                res = (*it).st;
            }
            if(it != segments[i].begin()){
                it--;
                if(y <= (*it).nd)res = y;
            }
            return res;
        };
        vector<int> v;
        vector<int> last = p;
        last[0] = 1;
        do{
            int diff = 0;
            while(p[diff] == last[diff])diff++;
            while(sz(v) > diff){
                tree.add(v.back(), v.back() + a - 1, 1);
                v.pop_back();
            }
            int pos = 0;
            if(sz(v) > 0)pos = v.back();
            fwd(i, sz(v), n){
                int x = p[i];
                pos = nextPos(pos, x);
                if(pos != -1){
                    v.push_back(pos);
                    tree.add(pos, pos + a - 1, -1);
                }else{
                    break;
                }
            }
            deb(p);
            deb(v);
            if(sz(v) == n)return true;
            last = p;
        }while(next_permutation(all(p)));
        return false;
    };
    int l = 0;
    int r = sz(frac) - 1;
    while(l < r){
        int m = (l + r) / 2 + 1;
        if(check(frac[m])){
            l = m;
        }else{
            r = m-1;
        }
    }
    cout << frac[l].st << "/" << frac[l].nd<< endl;
}

int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout << fixed << setprecision(10);
    solve();
    return 0;
}