#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")

#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/rope>

using namespace std;
using namespace __gnu_pbds;
using namespace __gnu_cxx;
 
typedef long long ll;
typedef long double ld;
typedef complex<ld> cd;

typedef pair<int, int> pi;
typedef pair<ll,ll> pl;
typedef pair<ld,ld> pd;

typedef vector<int> vi;
typedef vector<ld> vd;
typedef vector<ll> vl;
typedef vector<pi> vpi;
typedef vector<pl> vpl;
typedef vector<cd> vcd;

template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>;

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define F0R(i, a) for (int i = 0; i < (a); i++)
#define FORd(i,a,b) for (int i = (b)-1; i >= (a); i--)
#define F0Rd(i,a) for (int i = (a)-1; i >= 0; i--)
#define trav(a, x) for (auto& a : x)

#define mp make_pair
#define pb push_back
#define f first
#define s second
#define lb lower_bound
#define ub upper_bound

#define sz(x) (int)x.size()
#define beg(x) x.begin()
#define en(x) x.end()
#define all(x) beg(x), en(x)
#define resz resize

const int MOD = 1000000007; // 998244353
const ll INF = 1e18;
const int MX = 100001;
const ld PI = 4*atan((ld)1);

template<class T> void ckmin(T &a, T b) { a = min(a, b); }
template<class T> void ckmax(T &a, T b) { a = max(a, b); }

namespace input {
    template<class T> void re(complex<T>& x);
    template<class T1, class T2> void re(pair<T1,T2>& p);
    template<class T> void re(vector<T>& a);
    template<class T, size_t SZ> void re(array<T,SZ>& a);

    template<class T> void re(T& x) { cin >> x; }
    void re(double& x) { string t; re(t); x = stod(t); }
    void re(ld& x) { string t; re(t); x = stold(t); }
    template<class Arg, class... Args> void re(Arg& first, Args&... rest) { 
        re(first); re(rest...); 
    }

    template<class T> void re(complex<T>& x) { T a,b; re(a,b); x = cd(a,b); }
    template<class T1, class T2> void re(pair<T1,T2>& p) { re(p.f,p.s); }
    template<class T> void re(vector<T>& a) { F0R(i,sz(a)) re(a[i]); }
    template<class T, size_t SZ> void re(array<T,SZ>& a) { F0R(i,SZ) re(a[i]); }
}

using namespace input;

namespace output {
    template<class T1, class T2> void pr(const pair<T1,T2>& x);
    template<class T, size_t SZ> void pr(const array<T,SZ>& x);
    template<class T> void pr(const vector<T>& x);
    template<class T> void pr(const set<T>& x);
    template<class T1, class T2> void pr(const map<T1,T2>& x);

    template<class T> void pr(const T& x) { cout << x; }
    template<class Arg, class... Args> void pr(const Arg& first, const Args&... rest) { 
        pr(first); pr(rest...); 
    }

    template<class T1, class T2> void pr(const pair<T1,T2>& x) { 
        pr("{",x.f,", ",x.s,"}"); 
    }
    template<class T> void prContain(const T& x) {
        pr("{");
        bool fst = 1; trav(a,x) pr(!fst?", ":"",a), fst = 0; 
        pr("}");
    }
    template<class T, size_t SZ> void pr(const array<T,SZ>& x) { prContain(x); }
    template<class T> void pr(const vector<T>& x) { prContain(x); }
    template<class T> void pr(const set<T>& x) { prContain(x); }
    template<class T1, class T2> void pr(const map<T1,T2>& x) { prContain(x); }
    
    void ps() { pr("\n"); } 
    template<class Arg, class... Args> void ps(const Arg& first, const Args&... rest) { 
        pr(first," "); ps(rest...); // print w/ spaces
    }
}

using namespace output;

namespace io {
    void setIn(string s) { freopen(s.c_str(),"r",stdin); }
    void setOut(string s) { freopen(s.c_str(),"w",stdout); }
    void setIO(string s = "") {
        ios_base::sync_with_stdio(0); cin.tie(0); // fast I/O
        if (sz(s)) { setIn(s+".in"), setOut(s+".out"); } // for USACO
    }
}

using namespace io;

template<class T> T invGeneral(T a, T b) {
    a %= b; if (a == 0) return b == 1 ? 0 : -1;
    T x = invGeneral(b,a); 
    return x == -1 ? -1 : ((1-(ll)b*x)/a+b)%b;
}

template<class T> struct modular {
    T val; 
    explicit operator T() const { return val; }
    modular() { val = 0; }
    template<class U> modular(const U& v) {
        val = (-MOD <= v && v <= MOD) ? v : v % MOD;
        if (val < 0) val += MOD;
    }
    friend ostream& operator<<(ostream& os, const modular& a) { return os << a.val; }
    friend bool operator==(const modular& a, const modular& b) { return a.val == b.val; }
    friend bool operator!=(const modular& a, const modular& b) { return !(a == b); }

    modular operator-() const { return modular(-val); }
    modular& operator+=(const modular& m) { if ((val += m.val) >= MOD) val -= MOD; return *this; }
    modular& operator-=(const modular& m) { if ((val -= m.val) < 0) val += MOD; return *this; }
    modular& operator*=(const modular& m) { val = (ll)val*m.val%MOD; return *this; }
    friend modular exp(modular a, ll p) {
        modular ans = 1; for (; p; p /= 2, a *= a) if (p&1) ans *= a;
        return ans;
    }
    friend modular inv(const modular& a) { return invGeneral(a.val,MOD); } 
    // inv is equivalent to return exp(b,b.mod-2) if prime
    modular& operator/=(const modular& m) { return (*this) *= inv(m); }
    
    friend modular operator+(modular a, const modular& b) { return a += b; }
    friend modular operator-(modular a, const modular& b) { return a -= b; }
    friend modular operator*(modular a, const modular& b) { return a *= b; }
    
    friend modular operator/(modular a, const modular& b) { return a /= b; }
};

typedef modular<int> mi;
typedef pair<mi,mi> pmi;
typedef vector<mi> vmi;
typedef vector<pmi> vpmi;

int H,W;
int cum[186][186];
array<array<array<int,186>,186>,186> r,c,R,C;
string A[185];

int getSum(int r0, int r1, int c0, int c1) {
    return cum[r1+1][c1+1]-cum[r0][c1+1]-cum[r1+1][c0]+cum[r0][c0];
}

bool ok(int r0, int r1, int c0, int c1) {
    int x = getSum(r0,r1,c0,c1);
    return x == 0 || x == (r1-r0+1)*(c1-c0+1);
}

void triRect(int r0, int r1, int c0, int c1) {
    if (r0 > r1 || c0 > c1) return;
    ckmax(R[r0][r1][c0],c1+1);
    ckmax(C[c0][c1][r0],r1+1);
}

int main() {
    setIO(); re(H,W);
    F0R(i,H) {
        re(A[i]);
        F0R(j,W) cum[i+1][j+1] = (A[i][j]=='#')+cum[i+1][j]+cum[i][j+1]-cum[i][j];
    }
    F0Rd(k,W+1) F0R(i,H) FOR(j,i,H) {
        r[i][j][k] = k;
        if (k < W && ok(i,j,k,k)) {
            r[i][j][k] = k+1;
            if (ok(i,j,k,r[i][j][k+1]-1)) r[i][j][k] = r[i][j][k+1];
        }
        // ps("AA",i,j,k,ok(i,j,k,k),r[i][j][k]);
    }
    F0Rd(k,H+1) F0R(i,W) FOR(j,i,W) {
        c[i][j][k] = k;
        if (k < H && ok(k,k,i,j)) {
            c[i][j][k] = k+1;
            if (ok(k,c[i][j][k+1]-1,i,j)) c[i][j][k] = c[i][j][k+1];
        }
        // ps("BB",i,j,k,c[i][j][k]);
    }
    int ans = 0;
    while (r[0][H-1][0] != W) {
        ans ++;
        R = array<array<array<int,186>,186>,186>();
        C = array<array<array<int,186>,186>,186>();
        F0R(i,H) FOR(j,i,H) F0R(k,W) triRect(i,j,k,r[i][j][r[i][j][k]]-1);
        F0R(i,W) FOR(j,i,W) F0R(k,H) triRect(k,c[i][j][c[i][j][k]]-1,i,j);
        swap(r,R); swap(c,C);
        F0R(k,W+1) F0R(i,H) FORd(j,i,H) {
            ckmax(r[i][j][k],k);
            if (i != j) {
                ckmax(r[i+1][j][k],r[i][j][k]);
                ckmax(r[i][j-1][k],r[i][j][k]);
            }
            if (k != W) ckmax(r[i][j][k+1],r[i][j][k]);
        }
        F0R(k,H+1) F0R(i,W) FORd(j,i,W) {
            ckmax(c[i][j][k],k);
            if (i != j) {
                ckmax(c[i+1][j][k],c[i][j][k]);
                ckmax(c[i][j-1][k],c[i][j][k]);
            }
            if (k != H) ckmax(c[i][j][k+1],c[i][j][k]);
        }
    }
    ps(ans);
}

/* stuff you should look for
    * int overflow, array bounds
    * special cases (n=1?), set tle
    * do smth instead of nothing and stay organized
*/


Measured memory_bytes: 101120