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#include <bits/stdc++.h>
using namespace std;

#define rep(i, a, b) for (int i = (a); i < (b); i++)
#define all(x) begin(x), end(x)
#define sz(x) int((x).size())
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;

#ifdef LOCAL
auto operator<<(auto& o, auto x) -> decltype(x.first, o);
auto operator<<(auto& o, auto x) -> decltype(x.end(), o) {
  o << "{";
  for (int i = 0; auto y : x) o << ", " + !i++ * 2 << y;
  return o << "}"; }
auto operator<<(auto& o, auto x) -> decltype(x.first, o) {
  return o << "(" << x.first << ", " << x.second << ")"; }
void __print(auto... x) { ((cerr << x << " "), ...) << endl; }
#define debug(x...) __print("[" #x "]:", x)
#else
#define debug(...) 2137
#endif

const int nax = 1e5 + 10;
const int N = 18;
int pref[N][nax];
int bnd[nax];
int n, m;
int cnt[N];

mt19937 rng(10);

struct frac {
  int a, b;
  bool operator<(const frac o) const {
    return a * o.b < o.a * b;
  }
};

bool check(vector<int>& ind, frac t) {
  int num = t.a;
  int den = t.b;
  fill(cnt, cnt + n, 0);
  queue<pair<int, int>> q1;
  stack<pair<int, int>> q2;
  int to_sub = 0;
  q1.push({0, bnd[0]});
  cnt[bnd[0]]++;
  q2.push({m * den, 0});
  for(int i = m - 1; i > 0; i--) q2.push({i * den, bnd[i]});
  bool ok = 1;
  for(int i = 0; i < n; i++) {
    int id = ind[i];
    while(!q2.empty()) {
      auto [cur, cur_val] = q1.front();
      int rt = cur + num;
      while(!q2.empty() && q2.top().first < rt) {
        auto [j, val] = q2.top();
        q2.pop();
        q1.push({j, val + to_sub});
        cnt[val]++;
      }
      if(q2.empty()) break;
      if(cnt[0] || pref[id][(rt + den - 1) / den] - pref[id][cur / den] > 0) {
        q1.pop();
        cnt[cur_val - to_sub]--;
        if(q1.empty()) {
          auto [j, val] = q2.top();
          q2.pop();
          q1.push({j, val + to_sub});
          cnt[val]++;
        }
      } else break;
    }
    if(q2.empty()) return 0;

    for(int j = 0; j < n - 1; j++) cnt[j] = cnt[j + 1];
    cnt[n - 1] = 0;
    int cur = q1.front().first;
    int rt = cur + num;
    if(q2.top().first != rt) q2.push({rt, bnd[rt / den]});
    to_sub++;
  }
  return 1;
}

void solve() {
  cin >> n >> m;

  vector<string> v(n);
  for(int i = 0; i < n; i++) cin >> v[i];

  shuffle(all(v), rng);
  for(int i = 0; i < n; i++) {
    for(int j = 0; j < m; j++) {
      bnd[j] += v[i][j] == 'X';
      pref[i][j + 1] = pref[i][j] + (v[i][j] == 'X');
    }
  }
  for(int i = 0; i < m; i++) {
    bnd[i] = n - 1 - bnd[i];
    if(bnd[i] < 0) {
      cout << -1 << endl;
      return;
    }
  }

  frac ans{0, 1};
  vector<int> ind(n);
  iota(all(ind), 0);
  do {
    for(int mult = n; mult >= (n + 1) / 2; mult--) {
      if(mult == n) {
        int l = ans.a * mult / ans.b, r = mult * m + 1;
        while(l + 1 < r) {
          int mid = (l + r) >> 1;
          if(check(ind, {mid, mult})) l = mid;
          else r = mid;
        }
        ans = max(ans, {l, mult});
      } else {
        int l = (ans.a * mult + ans.b - 1) / ans.b;
        if(check(ind, {l, mult})) {
          ans = max(ans, {l, mult});
        }
      }
    }
  } while(next_permutation(all(ind)));

  auto [num, den] = ans;
  int g = gcd(num, den);
  num /= g;
  den /= g;
  cout << num << "/" << den << endl;
}

int main() {
  cin.tie(0)->sync_with_stdio(0);

  solve();
}