#include <bits/stdc++.h>
using namespace std;
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);i++)
#define REP(i, n) FOR(i,0,n)
#define IFOR(i, begin, end) for(int i=(end)-1;i>=(begin);i--)
#define IREP(i, n) IFOR(i,0,n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(),v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x) distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x) distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define Max(a, b) a = max(a, b)
#define Min(a, b) a = min(a, b)
#define bit(n) (1LL<<(n))
#define bit_exist(x, n) ((x >> n) & 1)
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v) cout << #v << "=" << endl; REP(i, v.size()){ cout << v[i] << ","; } cout << endl;
#define mdebug(m) cout << #m << "=" << endl; REP(i, m.size()){ REP(j, m[i].size()){ cout << m[i][j] << ","; } cout << endl;}
#define pb push_back
#define f first
#define s second
//#define int long long
#define INF 1000000000
template<typename T> istream &operator>>(istream &is, vector<T> &v){ for (auto &x : v) is >> x; return is; }
template<typename T> ostream &operator<<(ostream &os, vector<T> &v){ for(int i = 0; i < v.size(); i++) { cout << v[i]; if(i != v.size() - 1) cout << endl; }; return os; }
template<typename T> void Out(T x) { cout << x << endl; }
template<typename T1, typename T2> void Ans(bool f, T1 y, T2 n) { if(f) Out(y); else Out(n); }

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using PiP = pair<int, Pii>;
using PPi = pair<Pii, int>;
using bools = vector<bool>;
using pairs = vector<Pii>;

//int dx[4] = {1,0,-1,0};
//int dy[4] = {0,1,0,-1};
//char d[4] = {'D','R','U','L'};

const int mod = 1000000007;
//const int mod = 998244353;
#define Add(x, y) x = (x + (y)) % mod
#define Mult(x, y) x = (x * (y)) % mod

struct SegmentTree
{
    using T = int;

    int N;
    vector<T> dat;

    T id = -INF;
    T F(T a, T b) { return max(a, b); }
    
    SegmentTree(int n){
        N = 1;
        while(n > N) N = N << 1;
        dat = vector<T>(2 * N - 1, id);
    }

    SegmentTree(int n, vector<T> v){
        N = 1;
        while(n > N) N = N << 1;
        dat = vector<T>(2 * N - 1, id);
        for(int i = 0; i < n; i++) dat[i + N - 1] = v[i];
        for(int i = N - 2; i >= 0; i--) dat[i] = F(dat[i * 2 + 1], dat[i * 2 + 2]); 
    }

    void initialize(vector<T> &v){
        for(int i = 0; i < v.size(); i++) dat[i + N - 1] = v[i];
        for(int i = N - 2; i >= 0; i--) dat[i] = F(dat[i * 2 + 1], dat[i * 2 + 2]); 
    }

    void update(int k, T a){
        k += N - 1;
        dat[k] = a;
        while(k > 0){
            k = (k - 1) / 2;
            dat[k] = F(dat[k * 2 + 1], dat[k * 2 + 2]);
        }
    }

    void reset() { fill(dat.begin(), dat.end(), id); }

    T get(int a, int b, int k, int l, int r){
        if(r <= a || b <= l) return id;
        if(a <= l && r <= b) return dat[k];
        else{
            T vl = get(a, b, k * 2 + 1, l, (l + r) / 2);
            T vr = get(a, b, k * 2 + 2, (l + r) / 2, r);
            return F(vl, vr);
        }
    }
    T get(int a, int b) { return get(a, b, 0, 0, N); }

    T val(int k){ return dat[k + N - 1]; }
};

int calcsum(int x1, int x2, int y1, int y2, mat &S){
    return S[x2 + 1][y2 + 1] - S[x2 + 1][y1] - S[x1][y2 + 1] + S[x1][y1];
}

signed main(){

    int H, W; cin >> H >> W;
    vector<string> A(H); cin >> A;

    mat S(H + 1, vec(W + 1, 0));
    REP(i, H) REP(j, W) if(A[i][j] == '#') S[i + 1][j + 1]++;
    REP(i, H + 1) REP(j, W) S[i][j + 1] += S[i][j];
    REP(j, W + 1) REP(i, H) S[i + 1][j] += S[i][j];
    //mdebug(S);

    int M = 16;
    vector<mat> dp1(H, mat(H, vec(W, -INF))), dp2(W, mat(W, vec(H, -INF)));
    REP(i1, H) FOR(i2, i1, H) REP(j, W){
        int v = 0;
        if(A[i1][j] == '#') v = 1;
        if(calcsum(i1, i2, j, j, S) == v * (i2 - i1 + 1)){
            int x1 = j, x2 = W;
            while(x2 - x1 > 1){
                int x = (x1 + x2) / 2;
                if(calcsum(i1, i2, j, x, S) == v * (i2 - i1 + 1) * (x - j + 1)) x1 = x;
                else x2 = x;
            }
            dp1[i1][i2][j] = x1;
        }
    }
    REP(j1, W) FOR(j2, j1, W) REP(i, H){
        int v = 0;
        if(A[i][j1] == '#') v = 1;
        if(calcsum(i, i, j1, j2, S) == v * (j2 - j1 + 1)){
            int x1 = i, x2 = H;
            while(x2 - x1 > 1){
                int x = (x1 + x2) / 2;
                if(calcsum(i, x, j1, j2, S) == v * (j2 - j1 + 1) * (x - i + 1)) x1 = x;
                else x2 = x;
            }
            dp2[j1][j2][i] = x1;
        }
    }
    if(dp1[0][H - 1][0] == W - 1 || dp2[0][W - 1][0] == H - 1){
        Out(0);
        return 0;
    }
    
    //SegmentTree ST1(W), ST2(H);
    vec M1(W), M2(H);
    FOR(k, 1, M + 1){
        vector<mat> dq1(H, mat(H, vec(W, -INF))), dq2(W, mat(W, vec(H, -INF)));
        REP(i1, H) FOR(i2, i1, H){
            /*
            ST1.initialize(dp1[i1][i2]);

            REP(j, W) if(dp1[i1][i2][j] >= 0){
                int tmp = ST1.get(j, dp1[i1][i2][j] + 2);
                Max(dq1[i1][i2][j], tmp);
            }*/
            M1[0] = dp1[i1][i2][0];
            FOR(j, 1, W) M1[j] = max(M1[j - 1], dp1[i1][i2][j]);
            REP(j, W) if(dp1[i1][i2][j] >= 0) Max(dq1[i1][i2][j], M1[min(W - 1, dp1[i1][i2][j] + 1)]);
        }

        REP(j1, W) FOR(j2, j1, W){
            /*
            ST2.initialize(dp2[j1][j2]);

            REP(i, H) if(dp2[j1][j2][i] >= 0){
                int tmp = ST2.get(i, dp2[j1][j2][i] + 2);
                Max(dq2[j1][j2][i], tmp);
            }*/
            M2[0] = dp2[j1][j2][0];
            FOR(i, 1, H) M2[i] = max(M2[i - 1], dp2[j1][j2][i]);
            REP(i, H) if(dp2[j1][j2][i] >= 0) Max(dq2[j1][j2][i], M2[min(H - 1, dp2[j1][j2][i] + 1)]);
        }
        dp1.swap(dq1); dp2.swap(dq2);
        REP(i1, H) FOR(i2, i1, H){
            int jmax = 0;
            REP(j, W){
                if(dp2[j][j][i1] < i2) continue;
                if(jmax < j) jmax = j;
                while(jmax < W - 1 && dp2[j][jmax + 1][i1] >= i2) jmax++;
                Max(dp1[i1][i2][j], jmax);
            }
        }
            
        REP(j1, W) FOR(j2, j1, W){
            int imax = 0;
            REP(i, H){
                if(dp1[i][i][j1] < j2) continue;
                if(imax < i) imax = i;
                while(imax < H - 1 && dp1[i][imax + 1][j1] >= j2) imax++;
                Max(dp2[j1][j2][i], imax);
            }
        }

        if(dp1[0][H - 1][0] == W - 1 || dp2[0][W - 1][0] == H - 1){
            Out(k);
            return 0;
        }
    }

    return 0;
}


Measured memory_bytes: 106568