#pragma region Macros
#pragma GCC optimize("O3")
#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define rep2(i, a, b) for(ll i = a; i <= b; ++i)
#define rep(i, n) for(ll i = 0; i < n; ++i)
#define rep3(i, a, b) for(ll i = a; i >= b; --i)
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pb push_back
#define eb emplace_back
#define vi vector<int>
#define vll vector<ll>
#define vpi vector<pii>
#define vpll vector<pll>
#define overload2(_1, _2, name, ...) name
#define vec(type, name, ...) vector<type> name(__VA_ARGS__)
#define VEC(type, name, size)                                                                                                                                  \
    vector<type> name(size);                                                                                                                                   \
    IN(name)
#define vv(type, name, h, ...) vector<vector<type>> name(h, vector<type>(__VA_ARGS__))
#define VV(type, name, h, w)                                                                                                                                   \
    vector<vector<type>> name(h, vector<type>(w));                                                                                                             \
    IN(name)
#define vvv(type, name, h, w, ...) vector<vector<vector<type>>> name(h, vector<vector<type>>(w, vector<type>(__VA_ARGS__)))
#define fi first
#define se second
#define all(c) begin(c), end(c)
#define ios ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
#define lb(c, x) distance((c).begin(), lower_bound(all(c), (x)))
#define ub(c, x) distance((c).begin(), upper_bound(all(c), (x)))
using namespace std;
template <class T> using pq = priority_queue<T>;
template <class T> using pqg = priority_queue<T, vector<T>, greater<T>>;
#define Size(c) (int)(c).size()
#define INT(...)                                                                                                                                               \
    int __VA_ARGS__;                                                                                                                                           \
    IN(__VA_ARGS__)
#define LL(...)                                                                                                                                                \
    ll __VA_ARGS__;                                                                                                                                            \
    IN(__VA_ARGS__)
#define ULL(...)                                                                                                                                               \
    ull __VA_ARGS__;                                                                                                                                           \
    IN(__VA_ARGS__)
#define STR(...)                                                                                                                                               \
    string __VA_ARGS__;                                                                                                                                        \
    IN(__VA_ARGS__)
#define CHR(...)                                                                                                                                               \
    char __VA_ARGS__;                                                                                                                                          \
    IN(__VA_ARGS__)
#define DBL(...)                                                                                                                                               \
    double __VA_ARGS__;                                                                                                                                        \
    IN(__VA_ARGS__)
#define LD(...)                                                                                                                                                \
    ld __VA_ARGS__;                                                                                                                                            \
    IN(__VA_ARGS__)
int scan() { return getchar(); }
template <class T> void scan(T a) { cin >> a; }
void scan(int &a) { cin >> a; }
void scan(long long &a) { cin >> a; }
void scan(char &a) { cin >> a; }
void scan(double &a) { cin >> a; }
void scan(long double &a) { cin >> a; }
void scan(char a[]) { scanf("%s", a); }
void scan(string &a) { cin >> a; }
template <class T> void scan(vector<T> &);
template <class T, size_t size> void scan(array<T, size> &);
template <class T, class L> void scan(pair<T, L> &);
template <class T, size_t size> void scan(T (&)[size]);
template <class T> void scan(vector<T> &a) {
    for(auto &i : a) scan(i);
}
template <class T> void scan(deque<T> &a) {
    for(auto &i : a) scan(i);
}
template <class T, size_t size> void scan(array<T, size> &a) {
    for(auto &i : a) scan(i);
}
template <class T, class L> void scan(pair<T, L> &p) {
    scan(p.first);
    scan(p.second);
}
template <class T, size_t size> void scan(T (&a)[size]) {
    for(auto &i : a) scan(i);
}
template <class T> void scan(T &a) { cin >> a; }
void IN() {}
template <class Head, class... Tail> void IN(Head &head, Tail &... tail) {
    scan(head);
    IN(tail...);
}
string stin() {
    string s;
    cin >> s;
    return s;
}
template <class T, class S> inline bool chmax(T &a, S b) {

    if(a < b) {
        a = b;
        return 1;
    }
    return 0;
}
template <class T, class S> inline bool chmin(T &a, S b) {

    if(a > b) {
        a = b;
        return 1;
    }
    return 0;
}
vi iota(int n) {
    vi a(n);
    iota(all(a), 0);
    return a;
}
template <class T> void UNIQUE(vector<T> &x) {
    sort(all(x));
    x.erase(unique(all(x)), x.end());
}
int in() {
    int x;
    cin >> x;
    return x;
}
ll lin() {
    unsigned long long x;
    cin >> x;
    return x;
}
void print() { putchar(' '); }
void print(bool a) { cout << a; }
void print(int a) { cout << a; }
void print(long long a) { cout << a; }
void print(char a) { cout << a; }
void print(string &a) { cout << a; }
void print(double a) { cout << a; }
template <class T> void print(const vector<T> &);
template <class T, size_t size> void print(const array<T, size> &);
template <class T, class L> void print(const pair<T, L> &p);
template <class T, size_t size> void print(const T (&)[size]);
template <class T> void print(const vector<T> &a) {

    if(a.empty()) return;
    print(a[0]);
    for(auto i = a.begin(); ++i != a.end();) {
        cout << " ";
        print(*i);
    }
    cout << endl;
}
template <class T> void print(const deque<T> &a) {

    if(a.empty()) return;
    print(a[0]);
    for(auto i = a.begin(); ++i != a.end();) {
        cout << " ";
        print(*i);
    }
}
template <class T, size_t size> void print(const array<T, size> &a) {
    print(a[0]);
    for(auto i = a.begin(); ++i != a.end();) {
        cout << " ";
        print(*i);
    }
}
template <class T, class L> void print(const pair<T, L> &p) {
    cout << '(';
    print(p.first);
    cout << ",";
    print(p.second);
    cout << ')';
}
template <class T> void print(set<T> &x) {
    for(auto e : x) print(e), cout << " ";
    cout << endl;
}
template <class T> void print(multiset<T> &x) {
    for(auto e : x) print(e), cout << " ";
    cout << endl;
}
template <class T, size_t size> void print(const T (&a)[size]) {
    print(a[0]);
    for(auto i = a; ++i != end(a);) {
        cout << " ";
        print(*i);
    }
}
template <class T> void print(const T &a) { cout << a; }
int out() {
    putchar('\n');
    return 0;
}
template <class T> int out(const T &t) {
    print(t);
    putchar('\n');
    return 0;
}
template <class Head, class... Tail> int out(const Head &head, const Tail &... tail) {
    print(head);
    putchar(' ');
    out(tail...);
    return 0;
}
ll gcd(ll a, ll b) {
    while(b) {
        ll c = b;
        b = a % b;
        a = c;
    }
    return a;
}
ll lcm(ll a, ll b) {

    if(!a || !b) return 0;
    return a * b / gcd(a, b);
}
vector<pll> factor(ll x) {
    vector<pll> ans;
    for(ll i = 2; i * i <= x; i++)

        if(x % i == 0) {
            ans.push_back({i, 1});
            while((x /= i) % i == 0) ans.back().second++;
        }

    if(x != 1) ans.push_back({x, 1});
    return ans;
}
template <class T> vector<T> divisor(T x) {
    vector<T> ans;
    for(T i = 1; i * i <= x; i++)

        if(x % i == 0) {
            ans.pb(i);

            if(i * i != x) ans.pb(x / i);
        }
    return ans;
}
template <typename T> void zip(vector<T> &x) {
    vector<T> y = x;
    sort(all(y));
    for(int i = 0; i < x.size(); ++i) { x[i] = lb(y, x[i]); }
}
int popcount(ll x) { return __builtin_popcountll(x); }
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int rnd(int n) { return uniform_int_distribution<int>(0, n - 1)(rng); }
template <typename T> void shuffle(vector<T> &v) {
    rep3(i, v.size() - 1, 1) { swap(v[i], v[rnd(i)]); }
}
#define endl '\n'
vector<string> YES{"NO", "YES"};
vector<string> Yes{"No", "Yes"};
vector<string> yes{"no", "yes"};

#ifdef _LOCAL
#undef endl
#define debug(x)                                                                                                                                               \
    cout << #x << ": ";                                                                                                                                        \
    print(x);                                                                                                                                                  \
    cout << endl;
void err() {}
template <class T> void err(const T &t) {
    print(t);
    cout << " ";
}
template <class Head, class... Tail> void err(const Head &head, const Tail &... tail) {
    print(head);
    putchar(' ');
    out(tail...);
}
#else
#define debug(x)
template <class... T> void err(const T &...) {}
#endif

template <typename T> struct edge {
    int from, to;
    T cost;
    int id;
    edge(int to, T cost) : from(-1), to(to), cost(cost) {}
    edge(int from, int to, T cost) : from(from), to(to), cost(cost) {}
    edge(int from, int to, T cost, int id) : from(from), to(to), cost(cost), id(id) {}
    edge &operator=(const int &x) {
        to = x;
        return *this;
    }
    operator int() const { return to; }
};
template <typename T> using Edges = vector<edge<T>>;

struct Graph : vector<vector<int>> {
    using vector<vector<int>>::vector;
    Graph(int n, int m) : vector(n) { read(m); }
    inline void add(int a, int b, bool directed = false) {
        (*this)[a].emplace_back(b);

        if(!directed) (*this)[b].emplace_back(a);
    }
    void read(int n = -1, int offset = 1, bool directed = false) {

        if(n == -1) n = this->size() - 1;
        int a, b;
        while(n--) {
            cin >> a >> b;
            Graph::add(a - offset, b - offset, directed);
        }
    }
};
template <typename T> struct WeightedGraph : vector<Edges<T>> {
    using vector<Edges<T>>::vector;
    inline void add(int a, int b, T c, bool directed = false) {
        (*this)[a].emplace_back(b, c);

        if(!directed) (*this)[b].emplace_back(a, c);
    }
    void read(int n = -1, int offset = 1) {

        if(n == -1) n = this->size() - 1;
        int a, b;
        T c;
        while(n--) {
            cin >> a >> b >> c;
            WeightedGraph::add(a - offset, b - offset, c);
        }
    }
};
struct Setup_io {
    Setup_io() {
        ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0);
        cout << fixed << setprecision(15);
    }
} setup_io;
#pragma endregion

// _________コーディングはここから！！___________
template <class T> struct CumulativeSum2D {
    vector<vector<T>> data;

    CumulativeSum2D(int W, int H) : data(W + 1, vector<int>(H + 1, 0)) {}

    void add(int x, int y, T z) {
        ++x, ++y;
        if(x >= data.size() || y >= data[0].size()) return;
        data[x][y] += z;
    }

    void build() {
        for(int i = 1; i < data.size(); i++) {
            for(int j = 1; j < data[i].size(); j++) { data[i][j] += data[i][j - 1] + data[i - 1][j] - data[i - 1][j - 1]; }
        }
    }

    T query(int sx, int sy, int gx, int gy) { return (data[gx][gy] - data[sx][gy] - data[gx][sy] + data[sx][sy]); }
};
main() {
    INT(h, w);
    VV(char, S, h, w);
    // vv(char, S, h, w);
    // rep(i, h) rep(j, w) if(rnd(2)) S[i][j] = '#';
    // rep(i, h) {
    //     rep(j, w) cout << S[i][j];
    //     cout << endl;
    // }
    CumulativeSum2D<int> cum(h, w);
    rep(i, h) rep(j, w) cum.add(i, j, S[i][j] == '#');
    cum.build();
    vvv(int, a, h + 1, w + 1, h + 1);
    vvv(int, b, h + 1, w + 1, w + 1);
    rep(i, h + 1) rep(j, w + 1) {
        rep2(ii, i + 1, h) {
            int jj = j;
            while(jj < w) {
                int c = cum.query(i, j, ii, jj + 1);
                if(!c or c == (ii - i) * (jj + 1 - j))
                    jj++;
                else
                    break;
            }
            a[i][j][ii] = jj;
        }
        rep2(jj, j + 1, w) {
            int ii = i;
            while(ii < h) {
                int c = cum.query(i, j, ii + 1, jj);
                if(!c or c == (ii + 1 - i) * (jj - j))
                    ii++;
                else
                    break;
            }
            b[i][j][jj] = ii;
        }
    }
    rep(ans, 1000) {
        if(a[0][0][h] == w or b[0][0][w] == h) {
            cout << ans << endl;
            break;
        }
        rep(i, h) rep(j, w) {
            rep2(ii, i + 1, h) a[i][j][ii] = a[i][a[i][j][ii]][ii];
            rep2(jj, j + 1, w) b[i][j][jj] = b[b[i][j][jj]][j][jj];
        }

        rep(i, h) rep(j, w) {
            rep2(ii, i + 1, h) chmax(b[i][j][a[i][j][ii]], (int)ii);
            rep2(jj, j + 1, w) chmax(a[i][j][b[i][j][jj]], (int)jj);
        }
        rep(i, h) rep(j, w) {
            rep3(ii, h - 1, i + 1) chmax(a[i][j][ii], a[i][j][ii + 1]);
            rep3(jj, w - 1, j + 1) chmax(b[i][j][jj], b[i][j][jj + 1]);
        }
    }
    // int dp[10][10][10][10] = {};
    // auto dfs = [&](auto &&f, int a, int b, int c, int d) -> int {
    //     if(dp[a][b][c][d]) return dp[a][b][c][d];
    //     if(cum.query(a, c, b, d) == 0 or cum.query(a, c, b, d) == (b - a) * (d - c)) return dp[a][b][c][d] = 1;
    //     dp[a][b][c][d] = INT_MAX;
    //     rep2(i, a + 1, b - 1) chmin(dp[a][b][c][d], max(f(f, a, i, c, d), f(f, i, b, c, d)) + 1);
    //     rep2(i, c + 1, d - 1) chmin(dp[a][b][c][d], max(f(f, a, b, c, i), f(f, a, b, i, d)) + 1);
    //     return dp[a][b][c][d];
    // };
    // cout << dfs(dfs, 0, h, 0, w) - 1 << endl;
    // cout << dp[0][3][0][5] << " " << dp[3][5][0][5] << endl;
    // cout << dp[0][3][0][3] << " " << dp[0][3][3][5] << endl;
}

Measured memory_bytes: 56248