#include <bits/stdc++.h>
#define ll long long
#define INF 1000000005
#define MOD 1000000007
#define EPS 1e-10
#define rep(i,n) for(int i=0;i<(int)(n);++i)
#define rrep(i,n) for(int i=(int)(n)-1;i>=0;--i)
#define srep(i,s,t) for(int i=(int)(s);i<(int)(t);++i)
#define each(a,b) for(auto& (a): (b))
#define all(v) (v).begin(),(v).end()
#define len(v) (int)(v).size()
#define zip(v) sort(all(v)),v.erase(unique(all(v)),v.end())
#define cmx(x,y) x=max(x,y)
#define cmn(x,y) x=min(x,y)
#define fi first
#define se second
#define pb push_back
#define show(x) cout<<#x<<" = "<<(x)<<endl
#define sar(a,n) {cout<<#a<<":";rep(pachico,n)cout<<" "<<a[pachico];cout<<endl;}

using namespace std;

template<typename S,typename T>auto&operator<<(ostream&o,pair<S,T>p){return o<<"{"<<p.fi<<","<<p.se<<"}";}
template<typename T>auto&operator<<(ostream&o,set<T>s){for(auto&e:s)o<<e<<" ";return o;}
template<typename S,typename T,typename U>
auto&operator<<(ostream&o,priority_queue<S,T,U>q){while(!q.empty())o<<q.top()<<" ",q.pop();return o;}
template<typename K,typename T>auto&operator<<(ostream&o,map<K,T>&m){for(auto&e:m)o<<e<<" ";return o;}
template<typename T>auto&operator<<(ostream&o,vector<T>v){for(auto&e:v)o<<e<<" ";return o;}
void ashow(){cout<<endl;}template<typename T,typename...A>void ashow(T t,A...a){cout<<t<<" ";ashow(a...);}
template<typename S,typename T,typename U>
struct TRI{S fi;T se;U th;TRI(){}TRI(S f,T s,U t):fi(f),se(s),th(t){}
bool operator<(const TRI&_)const{return(fi==_.fi)?((se==_.se)?(th<_.th):(se<_.se)):(fi<_.fi);}};
template<typename S,typename T,typename U>
auto&operator<<(ostream&o,TRI<S,T,U>&t){return o<<"{"<<t.fi<<","<<t.se<<","<<t.th<<"}";}

typedef pair<int, int> P;
typedef pair<ll, ll> pll;
typedef TRI<int, int, int> tri;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef vector<P> vp;
typedef vector<double> vd;
typedef vector<string> vs;

const int MAX_N = 100005;

// r[i][j][k][l]: complexity <= i, upper j, lower k, left l ⇒ right pos
short r[17][190][190][190], c[17][190][190][190];
short cpos[190][190], rpos[190][190];
int sm[190][190];
bool f[190][190];

bool full(int u, int l, int d, int r)
{
    const int res = sm[d][r] - sm[u][r] - sm[d][l] + sm[u][l];
    return (res == (d - u) * (r - l) || res == 0);
}

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    int h, w;
    cin >> h >> w;
    rep(i,h){
        string s;
        cin >> s;
        rep(j,w){
            f[i][j] = (s[j] == '#');
        }
    }
    rep(j,w){
        rep(i,h){
            sm[i+1][j+1] = sm[i][j+1] + f[i][j];
        }
    }
    rep(i,h){
        rep(j,w){
            sm[i+1][j+1] += sm[i+1][j];
        }
    }
    rep(j,h){
        srep(k,j+1,h+1){
            rep(l,w){
                if(full(j,l,k,w)){
                    r[0][j][k][l] = w;
                    continue;
                }
                int a = l, b = w;
                while(b-a>1){
                    int mid = (a + b) / 2;
                    if(full(j,l,k,mid)) a = mid;
                    else b = mid;
                }
                r[0][j][k][l] = a;
            }
        }
    }
    rep(j,w){
        srep(k,j+1,w+1){
            rep(l,h){
                if(full(l,j,h,k)){
                    c[0][j][k][l] = h;
                    continue;
                }
                int a = l, b = h;
                while(b-a>1){
                    int mid = (a + b) / 2;
                    if(full(l,j,mid,k)) a = mid;
                    else b = mid;
                }
                c[0][j][k][l] = a;
            }
        }
    }
    rep(i,16){
        rep(j,h) rep(l,w) cpos[j][l] = w;
        rep(j,w) rep(l,h) rpos[j][l] = h;
        rep(j,h){
            srep(k,j+1,h+1){
                rep(l,w){
                    r[i+1][j][k][l] = (r[i][j][k][l] == w) ? w : r[i][j][k][r[i][j][k][l]];
                    while(cpos[j][l] > l && ((c[i][l][cpos[j][l]][j] == h) ? h : c[i][l][cpos[j][l]][c[i][l][cpos[j][l]][j]]) < k) --cpos[j][l];
                    cmx(r[i+1][j][k][l], cpos[j][l]);
                }
            }
        }
        rep(j,w){
            srep(k,j+1,w+1){
                rep(l,h){
                    c[i+1][j][k][l] = (c[i][j][k][l] == h) ? h : c[i][j][k][c[i][j][k][l]];
                    while(rpos[j][l] > l && ((r[i][l][rpos[j][l]][j] == w) ? w : r[i][l][rpos[j][l]][r[i][l][rpos[j][l]][j]]) < k) --rpos[j][l];
                    cmx(c[i+1][j][k][l], rpos[j][l]);
                }
            }
        }
    }
    rep(i,17){
        if(r[i][0][h][0] == w || c[i][0][w][0] == h){
            cout << i << "\n";
            break;
        }
    }
    return 0;
}

Measured memory_bytes: 454912