#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef unsigned int uint;
typedef unsigned char uchar;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;

#define REP(i,x) for(int i=0;i<(int)(x);i++)
#define REPS(i,x) for(int i=1;i<=(int)(x);i++)
#define RREP(i,x) for(int i=((int)(x)-1);i>=0;i--)
#define RREPS(i,x) for(int i=((int)(x));i>0;i--)
#define FOR(i,c) for(__typeof((c).begin())i=(c).begin();i!=(c).end();i++)
#define RFOR(i,c) for(__typeof((c).rbegin())i=(c).rbegin();i!=(c).rend();i++)
#define ALL(container) (container).begin(), (container).end()
#define RALL(container) (container).rbegin(), (container).rend()
#define SZ(container) ((int)container.size())
#define mp(a,b) make_pair(a, b)
#define pb push_back
#define eb emplace_back
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );
#define __builtin_popcount __builtin_popcountll

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; }
template<class T> ostream& operator<<(ostream &os, const vector<T> &t) {
    os<<"["; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"]"; return os;
}
template<class T> ostream& operator<<(ostream &os, const set<T> &t) {
    os<<"{"; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"}"; return os;
}
template<class S, class T> ostream& operator<<(ostream &os, const map<S, T> &t) {
    os<<"{"; FOR(it,t) {if(it!=t.begin()) os<<","; os<<*it;} os<<"}"; return os;
}
template<class S, class T> ostream& operator<<(ostream &os, const pair<S,T> &t) { return os<<"("<<t.first<<","<<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> 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);}

const int INF = 1<<28;
const double EPS = 1e-8;
const int MOD = 1000000007;

const int W = 122;
int T, n, m;
int A[200][200];
char memo[186][186][W][W];
int solve(int min_y, int max_y, int min_x, int max_x) {
    int sum = A[max_y][max_x] - A[min_y][max_x] - A[max_y][min_x] + A[min_y][min_x];
    if (sum == 0 || sum == (max_y - min_y) * (max_x - min_x)) {
        return 0;
    }

    if (max_y - min_y < W && max_x - min_x < W && memo[max_y][max_x][max_y - min_y][max_x - min_x] != -1) {
        return memo[max_y][max_x][max_y - min_y][max_x - min_x];
    }

    int ans = MOD;
    {
        int l = min_y, r = max_y;
        while(l + 1 < r) {
            int mid = (l + r) / 2;
            int lc = solve(min_y, mid, min_x, max_x);
            int rc = solve(mid, max_y, min_x, max_x);
            ans = min(ans, max(lc, rc) + 1);
            if (min(lc, rc) + 1 >= ans) break;
            if (lc < rc) l = mid;
            else r = mid;
        }
    }
    {
        int l = min_x, r = max_x;
        while(l + 1 < r) {
            int mid = (l + r) / 2;
            int lc = solve(min_y, max_y, min_x, mid);
            int rc = solve(min_y, max_y, mid, max_x);
            ans = min(ans, max(lc, rc) + 1);
            if (min(lc, rc) + 1 >= ans) break;
            if (lc < rc) l = mid;
            else r = mid;
        }
    }
    if (max_y - min_y < W && max_x - min_x < W) {
        return memo[max_y][max_x][max_y - min_y][max_x - min_x] = ans;
    }
    return ans;
}

int main(int argc, char *argv[]){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    while(cin >> n >> m){
        memset(A, 0, sizeof(A));
        memset(memo, -1, sizeof(memo));
        REPS(i, n) {
            REPS(j, m) {
                char c;
                cin >> c;
                A[i][j] = c == '#';
                A[i][j] = A[i][j-1] + A[i-1][j] - A[i-1][j-1] + A[i][j];
            }
        }
        cout << solve(0, n, 0, m) << endl;
    }
    return 0;
}


Measured memory_bytes: 503296