#include <bits/stdc++.h>
using namespace std;
using lint = long long int;
using pint = pair<int, int>;
using plint = pair<lint, lint>;
struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define FOR(i, begin, end) for(int i=(begin),i##_end_=(end);i<i##_end_;i++)
#define IFOR(i, begin, end) for(int i=(end)-1,i##_begin_=(begin);i>=i##_begin_;i--)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
template<typename T> istream &operator>>(istream &is, vector<T> &vec){ for (auto &v : vec) is >> v; return is; }
template<typename T> ostream &operator<<(ostream &os, const vector<T> &vec){ os << "["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const deque<T> &vec){ os << "deq["; for (auto v : vec) os << v << ","; os << "]"; return os; }
template<typename T> ostream &operator<<(ostream &os, const set<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_set<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T> ostream &operator<<(ostream &os, const unordered_multiset<T> &vec){ os << "{"; for (auto v : vec) os << v << ","; os << "}"; return os; }
template<typename T1, typename T2> ostream &operator<<(ostream &os, const pair<T1, T2> &pa){ os << "(" << pa.first << "," << pa.second << ")"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
template<typename TK, typename TV> ostream &operator<<(ostream &os, const unordered_map<TK, TV> &mp){ os << "{"; for (auto v : mp) os << v.first << "=>" << v.second << ","; os << "}"; return os; }
template<typename T> void ndarray(vector<T> &vec, int len) { vec.resize(len); }
template<typename T, typename... Args> void ndarray(vector<T> &vec, int len, Args... args) { vec.resize(len); for (auto &v : vec) ndarray(v, args...); }
template<typename T> bool mmax(T &m, const T q) { if (m < q) {m = q; return true;} else return false; }
template<typename T> bool mmin(T &m, const T q) { if (m > q) {m = q; return true;} else return false; }
template<typename T1, typename T2> pair<T1, T2> operator+(const pair<T1, T2> &l, const pair<T1, T2> &r) { return make_pair(l.first + r.first, l.second + r.second); }
template<typename T1, typename T2> pair<T1, T2> operator-(const pair<T1, T2> &l, const pair<T1, T2> &r) { return make_pair(l.first - r.first, l.second - r.second); }
#define dbg(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ") " << __FILE__ << endl;
#define FI first
#define SE second
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((lint)(x).size())
#define POW2(n) (1LL << (n))

int H, W;
vector<string> A;

using vvv = vector<vector<vector<int>>>;
vvv dp1, dp2, dp1new, dp2new;

int main()
{
    cin >> H >> W;
    A.resize(H);
    cin >> A;
    ndarray(dp1, H, W, H);
    ndarray(dp2, H, W, W);

    IREP(x, H) IREP(y, W)
    {
        dp1[x][y].resize(H - x);
        dp2[x][y].resize(W - y);

        if (x + 1 < H and A[x][y] == A[x + 1][y]) dp2[x][y][0] = dp2[x + 1][y][0] + 1;
        else dp2[x][y][0] = 1;

        if (y + 1 < W and A[x][y] == A[x][y + 1]) dp1[x][y][0] = dp1[x][y + 1][0] + 1;
        else dp1[x][y][0] = 1;
    }

    FOR(h, 1, H) REP(x, H - h) REP(y, W)
    {
        if (A[x][y] != A[x + h][y]) dp1[x][y][h] = 0;
        else dp1[x][y][h] = min(dp1[x][y][h - 1], dp1[x + h][y][0]);
    }

    FOR(w, 1, W) REP(x, H) REP(y, W - w)
    {
        if (A[x][y] != A[x][y + w]) dp2[x][y][w] = 0;
        else dp2[x][y][w] = min(dp2[x][y][w - 1], dp2[x][y + w][0]);
    }

    int K = 0;
    while (dp1[0][0][H - 1] < W and dp2[0][0][W - 1] < H)
    {
        K++;
        dp1new = dp1, dp2new = dp2;

        REP(x, H) REP(y, W)
        {
            REP(h, H - x)
            {
                int r = y + dp1[x][y][h];
                if (r < W) r = r + dp1[x][r][h];
                mmax(dp1new[x][y][h], r - y);
                if (r - y > 0) mmax(dp2new[x][y][r - y - 1], h + 1);
            }
            REP(w, W - y)
            {
                int b = x + dp2[x][y][w];
                if (b < H) b = b + dp2[b][y][w];
                mmax(dp2new[x][y][w], b - x);
                if (b - x > 0) mmax(dp1new[x][y][b - x - 1], w + 1);
            }
        }

        swap(dp1, dp1new);
        swap(dp2, dp2new);
        REP(x, H) REP(y, W)
        {
            IREP(h, H - x - 1) mmax(dp1[x][y][h], dp1[x][y][h + 1]);
            IREP(w, W - y - 1) mmax(dp2[x][y][w], dp2[x][y][w + 1]);
        }
    }

    cout << K << endl;
}


Measured memory_bytes: 81664