#include <bits/stdc++.h>
 
using namespace std;
 
#define DEBUG(x) cerr<<#x<<": "<<x<<endl;
#define DEBUG_VEC(v) cerr<<#v<<":";for(int i=0;i<v.size();i++) cerr<<" "<<v[i]; cerr<<endl;
#define DEBUG_MAT(v) cerr<<#v<<endl;for(int i=0;i<v.size();i++){for(int j=0;j<v[i].size();j++) {cerr<<v[i][j]<<" ";}cerr<<endl;}
typedef long long ll;
//#define int ll
 
#define vi vector<int>
#define vl vector<ll>
#define vii vector< vector<int> >
#define vll vector< vector<ll> >
#define vs vector<string>
#define pii pair<int,int>
#define pis pair<int,string>
#define psi pair<string,int>
#define pll pair<ll,ll>
template<class S, class T> pair<S, T> operator+(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first + t.first, s.second + t.second); }
template<class S, class T> pair<S, T> operator-(const pair<S, T> &s, const pair<S, T> &t) { return pair<S, T>(s.first - t.first, s.second - t.second); }
template<class S, class T> ostream& operator<<(ostream& os, pair<S, T> p) { os << "(" << p.first << ", " << p.second << ")"; return os; }
#define X first
#define Y second
#define rep(i,n) for(int i=0;i<(n);i++)
#define rep1(i,n) for(int i=1;i<=(n);i++)
#define rrep(i,n) for(int i=(n)-1;i>=0;i--)
#define rrep1(i,n) for(int i=(n);i>0;i--)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define in(x, a, b) (a <= x && x < b)
#define all(c) c.begin(),c.end()
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a = b; return 1; } return 0; }
#define UNIQUE(v) v.erase(std::unique(v.begin(), v.end()), v.end());
const ll inf = 1000000001;
const ll INF = (ll)1e18 + 1;
const long double pi = 3.1415926535897932384626433832795028841971L;
#define Sp(p) cout<<setprecision(25)<< fixed<<p<<endl;
//int dx[4] = {1, 0, -1, 0}, dy[4] = {0, 1, 0, -1};
//int dx2[8] = { 1,1,0,-1,-1,-1,0,1 }, dy2[8] = { 0,1,1,1,0,-1,-1,-1 };
vi dx = {1, 0, -1, 0}, dy = {0, 1, 0, -1};
vi dx2 = { 1,1,0,-1,-1,-1,0,1 }, dy2 = { 0,1,1,1,0,-1,-1,-1 };
#define fio() cin.tie(0); ios::sync_with_stdio(false);
const ll MOD = 1000000007;
//const ll MOD = 998244353;
// #define mp make_pair
//#define endl '\n'


const int N = 187;
int sum[N][N];
int dp[N][N][N][18];
int h, w;

int solve(int u, int d, int l, int x) {
    if (dp[u][d][l][x] != -inf) return dp[u][d][l][x];
    if (x == 0) {
        int ok = l, ng = w + 1;
        while (ok + 1 < ng) {
            int mid = (ok + ng) / 2;
            int area = sum[d][mid] - sum[d][l] - sum[u][mid] + sum[u][l];
            if (area == 0 or area == (mid - l) * (d - u)) ok = mid;
            else ng = mid;
        }
        if (ok == l) dp[u][d][l][x] = -1;
        else dp[u][d][l][x] = ok;
        // DEBUG_VEC(vi({u, d, l, x}));
        // DEBUG(dp[u][d][l][x]);
        return ok;
    }

    dp[u][d][l][x] = -1;
    chmax(dp[u][d][l][x], solve(u, d, l, x - 1));
    int m = solve(u, d, l, x - 1);
    // DEBUG_VEC(vi({u, d, l, x}));
    // DEBUG(m);
    if (m > l) {
        int r = solve(u, d, m, x - 1);
        // DEBUG_VEC(vi({u, d, l, x}));
        // DEBUG(r);
        chmax(dp[u][d][l][x], r);
    }

    int x1 = u + 1, x2 = d - 1;
    while (x1 + 1 < x2) {
        int m = (x1 + x2) / 2;
        if (solve(u, m, l, x - 1) >= solve(m, d, l, x - 1)) x1 = m;
        else x2 = m;
    }
    for (int m = x1; m <= x2; m++) {
        int r = min(solve(u, m, l, x - 1), solve(m, d, l, x - 1));
        // DEBUG_VEC(vi({u, d, l, x, m}));
        // DEBUG(r);
        chmax(dp[u][d][l][x], r);
    }
    return dp[u][d][l][x];
}

signed main() {
    fio();
    cin >> h >> w;

    vii a(h, vi(w));
    rep (i, h) {
        rep (j, w) {
            char c;
            cin >> c;
            a[i][j] = (int)(c == '#');
        }
    }

    rep (i, h) {
        rep (j, w) {
            sum[i + 1][j + 1] = sum[i + 1][j] + a[i][j];
        }
    }
    rep (i, h) {
        rep (j, w + 1) {
            sum[i + 1][j] += sum[i][j];
        }
    }

    rep (i, h) {
        for (int j = i; j <= h; j++) {
            rep (k, w) {
                rep (l, 18) {
                    dp[i][j][k][l] = -inf;
                }
            }
        }
    }

    int ok = 18, ng = -1;
    while (ng + 1 < ok) {
        int mid = (ng + ok) / 2;
        int res = solve(0, h, 0, mid);
        if (res == w) ok = mid;
        else ng = mid;
    }
    // rep (i, 8) {
    //     cout << dp[1][3][1][i] << endl;
    // }
    cout << ok << endl;
}

Measured memory_bytes: 451072