#include <bits/stdc++.h>
// created [2020/01/29] 10:36:14
#pragma GCC diagnostic ignored "-Wsign-compare"
#pragma GCC diagnostic ignored "-Wsign-conversion"

using i32   = int32_t;
using i64   = int64_t;
using u32   = uint32_t;
using u64   = uint64_t;
using uint  = unsigned int;
using usize = std::size_t;
using ll    = long long;
using ull   = unsigned long long;
using ld    = long double;
template<typename T, usize n>
using arr = T (&)[n];
template<typename T, usize n>
using c_arr = const T (&)[n];
template<typename T>
using max_heap = std::priority_queue<T>;
template<typename T>
using min_heap = std::priority_queue<T, std::vector<T>, std::greater<T>>;
template<typename T> constexpr T popcount(const T u) { return u ? static_cast<T>(__builtin_popcountll(static_cast<u64>(u))) : static_cast<T>(0); }
template<typename T> constexpr T log2p1(const T u) { return u ? static_cast<T>(64 - __builtin_clzll(static_cast<u64>(u))) : static_cast<T>(0); }
template<typename T> constexpr T msbp1(const T u) { return log2p1(u); }
template<typename T> constexpr T lsbp1(const T u) { return __builtin_ffsll(u); }
template<typename T> constexpr T clog(const T u) { return u ? log2p1(u - 1) : static_cast<T>(u); }
template<typename T> constexpr bool ispow2(const T u) { return u and (static_cast<u64>(u) & static_cast<u64>(u - 1)) == 0; }
template<typename T> constexpr T ceil2(const T u) { return static_cast<T>(1) << clog(u); }
template<typename T> constexpr T floor2(const T u) { return u == 0 ? static_cast<T>(0) : static_cast<T>(1) << (log2p1(u) - 1); }
template<typename T> constexpr bool btest(const T mask, const usize ind) { return static_cast<bool>((static_cast<u64>(mask) >> ind) & static_cast<u64>(1)); }
template<typename T> void bset(T& mask, const usize ind) { mask |= (static_cast<T>(1) << ind); }
template<typename T> void breset(T& mask, const usize ind) { mask &= ~(static_cast<T>(1) << ind); }
template<typename T> void bflip(T& mask, const usize ind) { mask ^= (static_cast<T>(1) << ind); }
template<typename T> void bset(T& mask, const usize ind, const bool b) { (b ? bset(mask, ind) : breset(mask, ind)); }
template<typename T> constexpr T bcut(const T mask, const usize ind) { return ind == 0 ? static_cast<T>(0) : static_cast<T>((static_cast<u64>(mask) << (64 - ind)) >> (64 - ind)); }
template<typename T> bool chmin(T& a, const T& b) { return (a > b ? a = b, true : false); }
template<typename T> bool chmax(T& a, const T& b) { return (a < b ? a = b, true : false); }
constexpr unsigned int mod                  = 1000000007;
template<typename T> constexpr T inf_v      = std::numeric_limits<T>::max() / 4;
template<typename Real> constexpr Real pi_v = Real{3.141592653589793238462643383279502884};
auto mfp = [](auto&& f) { return [=](auto&&... args) { return f(f, std::forward<decltype(args)>(args)...); }; };

template<typename T>
T in()
{
    T v;
    return std::cin >> v, v;
}
template<typename T, typename Uint, usize n, usize i>
T in_v(typename std::enable_if<(i == n), c_arr<Uint, n>>::type) { return in<T>(); }
template<typename T, typename Uint, usize n, usize i>
auto in_v(typename std::enable_if<(i < n), c_arr<Uint, n>>::type& szs)
{
    const usize s = (usize)szs[i];
    std::vector<decltype(in_v<T, Uint, n, i + 1>(szs))> ans(s);
    for (usize j = 0; j < s; j++) { ans[j] = in_v<T, Uint, n, i + 1>(szs); }
    return ans;
}
template<typename T, typename Uint, usize n>
auto in_v(c_arr<Uint, n> szs) { return in_v<T, Uint, n, 0>(szs); }
template<typename... Types>
auto in_t() { return std::tuple<std::decay_t<Types>...>{in<Types>()...}; }
struct io_init
{
    io_init()
    {
        std::cin.tie(nullptr), std::ios::sync_with_stdio(false);
        std::cout << std::fixed << std::setprecision(20);
    }
    void clear()
    {
        std::cin.tie(), std::ios::sync_with_stdio(true);
    }
} io_setting;

int out() { return 0; }
template<typename T>
int out(const T& v) { return std::cout << v, 0; }
template<typename T>
int out(const std::vector<T>& v)
{
    for (usize i = 0; i < v.size(); i++) {
        if (i > 0) { std::cout << ' '; }
        out(v[i]);
    }
    return 0;
}
template<typename T1, typename T2>
int out(const std::pair<T1, T2>& v) { return out(v.first), std::cout << ' ', out(v.second), 0; }
template<typename T, typename... Args>
int out(const T& v, const Args... args) { return out(v), std::cout << ' ', out(args...), 0; }
template<typename... Args>
int outln(const Args... args) { return out(args...), std::cout << '\n', 0; }
template<typename... Args>
void outel(const Args... args) { return out(args...), std::cout << std::endl, 0; }
#    define SHOW(...) static_cast<void>(0)
constexpr ull TEN(const usize n) { return n == 0 ? 1ULL : TEN(n - 1) * 10ULL; }

template<typename T, typename Uint, usize n, usize i>
auto make_v(typename std::enable_if<(i == n), c_arr<Uint, n>>::type, const T& v = T{}) { return v; }
template<typename T, typename Uint, usize n, usize i>
auto make_v(typename std::enable_if<(i < n), c_arr<Uint, n>>::type szs, const T& v = T{})
{
    const usize s = (usize)szs[i];
    return std::vector<decltype(make_v<T, Uint, n, i + 1>(szs, v))>(s, make_v<T, Uint, n, i + 1>(szs, v));
}
template<typename T, typename Uint, usize n>
auto make_v(c_arr<Uint, n> szs, const T& t = T{}) { return make_v<T, Uint, n, 0>(szs, t); }
int main()
{
    const auto H = in<int>(), W = in<int>();
    const auto ass = in_v<std::string>({H});
    auto bss       = make_v({H, W}, 0);
    for (int i = 0; i < H; i++) {
        for (int j = 0; j < W; j++) { bss[i][j] = ass[i][j] == '.'; }
    }
    for (int i = 0; i < H; i++) {
        for (int j = 1; j < W; j++) { bss[i][j] += bss[i][j - 1]; }
    }
    for (int i = 1; i < H; i++) {
        for (int j = 0; j < W; j++) { bss[i][j] += bss[i - 1][j]; }
    }
    auto sum = [&](int xl, int xu, int yl, int yu) {
        int ans = bss[yu - 1][xu - 1];
        if (yl > 0) { ans -= bss[yl - 1][xu - 1]; }
        if (xl > 0) { ans -= bss[yu - 1][xl - 1]; }
        if (xl > 0 and yl > 0) { ans += bss[yl - 1][xl - 1]; }
        return ans;
    };
    constexpr int C = 100;
    auto nxs        = make_v({H, H + 1, W}, -1);
    auto nys        = make_v({W, W + 1, H}, -1);
    for (int yl = 0; yl < H; yl++) {
        for (int yu = yl + 1; yu <= H; yu++) {
            for (int xl = 0; xl < W; xl++) {
                int inf = xl, sup = W + 1;
                while (sup - inf > 1) {
                    const int mid                    = (inf + sup) / 2;
                    const int xu                     = mid;
                    const int wc                     = sum(xl, xu, yl, yu);
                    const int bc                     = (xu - xl) * (yu - yl) - wc;
                    (wc == 0 or bc == 0 ? inf : sup) = mid;
                }
                nxs[yl][yu][xl] = inf;
            }
        }
    }
    for (int xl = 0; xl < W; xl++) {
        for (int xu = xl + 1; xu <= W; xu++) {
            for (int yl = 0; yl < H; yl++) {
                int inf = yl, sup = H + 1;
                while (sup - inf > 1) {
                    const int mid                    = (inf + sup) / 2;
                    const int yu                     = mid;
                    const int wc                     = sum(xl, xu, yl, yu);
                    const int bc                     = (xu - xl) * (yu - yl) - wc;
                    (wc == 0 or bc == 0 ? inf : sup) = mid;
                }
                nys[xl][xu][yl] = inf;
            }
        }
    }
    if (nxs[0][H][0] >= W) { return outln(0); }
    for (int c = 1; c < C; c++) {
        auto xtmp = make_v({H, H + 1, W}, -1);
        auto ytmp = make_v({W, W + 1, H}, -1);
        for (int yl = 0; yl < H; yl++) {
            for (int yu = yl + 1; yu <= H; yu++) {
                for (int xl = 0; xl < W; xl++) {
                    const int nx = nxs[yl][yu][xl];
                    if (nx >= W) {
                        chmax(xtmp[yl][yu][xl], nx);
                        chmax(ytmp[xl][nx][yl], yu);
                    } else {
                        const int nnx = nxs[yl][yu][nx];
                        chmax(xtmp[yl][yu][xl], nnx);
                        chmax(ytmp[xl][nnx][yl], yu);
                    }
                }
            }
        }
        for (int xl = 0; xl < W; xl++) {
            for (int xu = xl + 1; xu <= W; xu++) {
                for (int yl = 0; yl < H; yl++) {
                    const int ny = nys[xl][xu][yl];
                    if (ny >= H) {
                        chmax(ytmp[xl][xu][yl], ny);
                        chmax(xtmp[yl][ny][xl], xu);
                    } else {
                        const int nny = nys[xl][xu][ny];
                        chmax(ytmp[xl][xu][yl], nny);
                        chmax(xtmp[yl][nny][xl], xu);
                    }
                }
            }
        }
        for (int yl = 0; yl < H; yl++) {
            for (int yu = H - 1; yu > yl; yu--) {
                for (int xl = 0; xl < W; xl++) {
                    chmax(xtmp[yl][yu][xl], xtmp[yl][yu + 1][xl]);
                }
            }
        }
        for (int yu = 1; yu <= H; yu++) {
            for (int yl = 1; yl < yu; yl++) {
                for (int xl = 0; xl < W; xl++) {
                    chmax(xtmp[yl][yu][xl], xtmp[yl - 1][yu][xl]);
                }
            }
        }
        for (int xl = 0; xl < W; xl++) {
            for (int xu = W - 1; xu > xl; xu--) {
                for (int yl = 0; yl < H; yl++) {
                    chmax(ytmp[xl][xu][yl], ytmp[xl][xu + 1][yl]);
                }
            }
        }
        for (int xu = 1; xu <= W; xu++) {
            for (int xl = 1; xl < xu; xl++) {
                for (int yl = 0; yl < H; yl++) {
                    chmax(ytmp[xl][xu][yl], ytmp[xl - 1][xu][yl]);
                }
            }
        }
        nxs = xtmp, nys = ytmp;
        if (nxs[0][H][0] >= W) { return outln(c); }
    }
    return 0;
}


Measured memory_bytes: 107076